Exclude everything but *.rb from git diff output

[rtrv]

Closes #41

[Envek]

But what about Gemfile, Rakefile, Guardfile and config.ru? Rubocop checks some ruby files without .rb extension out of the box.

You can check it yourself by executing in your project:

bundle exec rubocop -L | pcregrep -v '.*\.rb$'

[rtrv]

@Envek yep, you're right. How about a configurable list of excluded file extensions?