Add common extension options

[Alexandre Barret]

We don't need to know how to write the regex for certain files. We only need to pass the programming language instead.

We could add something like:

retest --lang ruby
retest --lang go
retest --lang js
retest --lang ruby go js <- would that work?

We should still keep the extension option retest --ext "\\.rb$" which gives the flexibility for edge cases.