Rename file.contains_regex to file.contains_regex_multiline

Renamed `file.contains_regex` to `file.contains_regex_multiline`, as that's how it's written to behave, and it it more intuitively names this way.  I also refactored every instance of `file.contains_regex` to `file.contains_regex_multiline` to keep existing behavior, even though some of the instances probably *should* be `file.contains_regex`

I added a new `file.contains_regex` that operates on single lines.  This is probably the version most people will want to use.

@s0undt3ch, everything look good?  IIRC, you wrote the original `contains_regex`.