Add new function find-files*

`find-files*` is like `find-files` but allows the use the use of a predicate function to select files instead of a regular expression. `find-files` has been redefined in terms of `find-files*`. Tests for both functions were also added. I've never used midje before so I hope everything looks in order there.

`find-files*` was the best name I could come up with, if there's `autodisagree`ment with the name I can change it. :smile: