Issue title: [Question] How to change path for the cache folder _bookdown_files?
 Issue body: I search for a way to put the `_bookdown_files` folder into a different temporary destination.
I found the relevant variable [aux_diro](https://github.com/rstudio/bookdown/blob/a4c97d82dc46454c0c4ba3df309413cdaae89709/R/render.R#L86) in the source.
I also found an [issue](https://github.com/rstudio/bookdown/issues/323#issuecomment-284624721) that suggests to use `cache.path` but that didn't work for me.
There is an `output-dir` option in `_bookdown.yml` but that's for the output files only.

Subquestion: Can I avoid having `_bookdown.yml` in my project root? 
I'd like to keep the root directory clean and I already failed to get `template.tex` out of the way.
Thank you very much for your effort and help!

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 Comments: 
 Comment 0: > I search for a way to put the _bookdown_files folder into a different temporary destination.

Can you give more details on what you want to do exactly ? 

Currently it seems indeed that there is no way to rename it or change location.

Let's note that bookdown is designed and works better when everything is in the root of the bookdown project, not in subdirs. 

> Subquestion: Can I avoid having _bookdown.yml in my project root?

You can put this file where you want, but you'll need to specify the path when rendering the book using `config_file` in `render_book`

But I think you won't be able to use the build pane in that case. 

> I'd like to keep the root directory clean 

What would work though is having your project root one folder above and have your bookdown project in one folder, with everything bookdown related relative to that root folder. Something like 
```
myproject
  |--- myproject.Rproj
  |--- README.md
  |--- bookdown
        |--- _bookdown.yml
        |--- _book
        |--- _bookdown_files
        |--- index.Rmd
        |--- ...
```

Your root project dir would be clean, RStudio build pane would be configured to use the `bookdown` folder instead of the root folder, but you would need to specificy the folder in functions: `bookdown38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5serve_book("bookdown")` or `xfun38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5in_dir("bookdown", bookdown38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5render_book("index.Rmd"))`

I am waiting for more information on your use case to decide what could be done here. Thanks  !
 Comment 1: Thank you for the layout suggestion. Theres's still a mix of book config files and book content files inside the bookdown folder.
> Can you give more details on what you want to do exactly ?

Well, I'd like to use multiple file sync and version control systems without dealing with configurations like `.gitignore`. Ultimately temporary files like `_bookdown_files` should not be kept under version control but inside unrelated temporary directories.

> You can put this file where you want, but you'll need to specify the path when rendering the book using config_file in render_book

That works but `bookdown38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5serve_book(config_file='book_config/_bookdown.yml')` leads to 
`Error in server_config(dir, ..., baseurl = baseurl) : unused argument config_file`

There's no use if it's only working for the `render_book()` function

I also tried to combine the `_bookdown.yml` options with the yaml header inside `"index.Rmd"` but that didn't work. 
And I still wonder how to edit pandoc's --template option to use a subfolder like `book_config` for the `template.tex` file.
 Comment 2: > Ultimately temporary files like _bookdown_files should not be kept under version control but inside unrelated temporary directories.

`_bookdown_files` is already a folder where files required to compile the book are store. It contains `_files` and `_cache` folders for the book. This should be somewhere. What location do you suggest ? 
`.gitignore` is aimed at ignoring file for git, so if you want to ignore some files from version control, .gitignore must be used. I don't see why not use it.

> That works but bookdown38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5serve_book(config_file='book_config/_bookdown.yml') leads to
Error in server_config(dir, ..., baseurl = baseurl) : unused argument config_file

This is currently track in https://github.com/rstudio/bookdown/issues/953

> I also tried to combine the _bookdown.yml options with the yaml header inside "index.Rmd" but that didn't work.

This is related to https://github.com/rstudio/bookdown/issues/921

> And I still wonder how to edit pandoc's --template option to use a subfolder like book_config for the template.tex file.

You should be able to set the path to the template file in `template = ` inside `pdf_book` function. If it does not work, please open a new issue.
 Comment 3: > What location do you suggest ?

The default location is perfectly fine. Personally I'd like to configure it to reside in a temporary directory that gets deleted by the OS once in a while.

> I don't see why not use it.

Some sync software ignore .gitignore so you have to configure each software separately. Adding a bookdown config option would be the easiest way.
 Comment 4: This old thread has been automatically locked. If you think you have found something related to this, please open a new issue by following the issue guide (<https://yihui.org/issue/>), and link to this old issue if necessary.
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