regex=False as default behavior, and expand regex disabling options
I was very happy to see pr #5879, allowing disabling the auto-regex feature for functions in the .str namespace. Doing something about this has been on my todo list for a while. Automatic promotion of strings to regular expressions is one of the two big points of difficulty for beginners using pandas (in my experience anyway). The most obvious problem comes up when you have literal dollar signs you wish to operate on.

I'd like to argue that the faster, less confusing `regex=False` be the default.

This is a trivial thing to do, and if folks are willing to accept it, I'm happy to submit a pull request.

I also ran into this problem when using unicode delimiters with the read_\* family of functions:

```
ParserWarning: Falling back to the 'python' engine because the 'c' engine does not support regex separators; you can avoid this warning by specifying engine='python'.
```

This is a another case where we're losing performance and increasing confusion. So, I'd like to expand this option elsewhere, and could submit this all as one pull request, or two (if it seems reasonable).
