Issue title: Underbar changed to dash in generated url
 Issue body: If content is of the form 
    foo_bar.md
then
    make html
generates URLs of the form
    foo-bar.html

Under-bar is silently changed to dash.

For SEO purposes, dash is preferred to under-bar; so, I agree that generated URLs should use dashes; however, there should be a warning. If there are internal links between pages, translating under-bar to dash will break these links. The warning should include an explanation that internal links using under-bar need to be changed to dash. Possibly a settings item to allow under-bar in URL should be included.

 Comments: 
 Comment 0: The URL is generated from the `"Title: foo bar"` at the top of `foo_bar.md` which is different to how some other static site generators work (e.g. [Jekyll](https://github.com/mojombo/jekyll)).

 Comment 1: As @rupert mentioned, the URL slug auto-generation is based on the post title — not the source file name — and is only done if you do not explicitly set the slug yourself. If you want a URL slug that's different from the auto-generated version, simply specify it via the following metadata in your post:

```
slug: foo_bar
```

Let us know if we can be of further assistance. Issue closed.

{'source': 'GithubIssues', 'repo_name_with_owner': 'getpelican/pelican', 'issue_id': 12281121, 'repo_stars': 11189.0}