Improve completion and scan folders only on first call to improve perfor...

- Autocompleting was taking too much time on my project ( about 3,5 seconds) due to the os.walk. On the first call to autocomplete, we still scan all directories, but save all "step_definitions" directories so that we know where to look for on next call (taking about 70 ms now)
- Improved the ruby_regexp to allow having parenthesis
- Improved unbraced_chunks to allow using non captured groups (?:content_of_the_group)