[TxMt] Add a RuboCop bundle to bundle index

[Stefan Daschek]

I think TextMate’s bundle index should include a RuboCop bundle.

Disclaimer: I wrote a RuboCop bundle myself, and there are at least 
three more that I know about:

  1) https://github.com/mrdougal/textmate2-rubocop
  2) https://github.com/goyox86/rubocop-tmbundle
  3) https://github.com/fazibear/Rubocop.tmbundle
  4) My bundle: https://github.com/noniq/RuboCop.tmbundle

1) and 2) seem stale (last commit in Nov resp. Dec 2014). 3) and 4) are 
actively maintained.

One of the differences between 3) and 4) is that 3) supports different 
gutter marks for different severities, while 4) only uses a generic 
warning mark. On the other hand, 3) seems to only ever work on the 
current file, while 4) uses the filebrowser selection (if any, similar 
to the Git bundle for example).

In 
https://github.com/textmate/ruby.tmbundle/pull/99#issuecomment-296400597 
ff. there was discussion if the Ruby’s bundle already existing “Validate 
Syntax” command should be changed / extended to use RuboCop (if present) 
– this could be an alternative to adding an “official” RuboCop bundle.

Happy to hear your opinions!

Stefan