Customisable patterns for DWIM

[OldhamMade]

In the projects I work on, test files are suffixed with _spec.py instead of _test.py as we follow more of a BDD format. Test names don't start with test_ but instead start with one of: ensure_, it_, must_, should_

It would be great if there was a way to append _spec.py to the list of files checked so we can customise the behaviour a little more.

[wbolster]

heh, this was one of my concerns in #55 (comment) (and #73).

i use projectile.el and its projectile-test-file-p helper for the autodiscovery, and it seems you're using the non-projectile code path.

anyway, i'll merge a pull request to improve this!