Add files command?

[AlexB52]

It's might be useful for people who do not want to run retest but still list all specs changed between a git branch.
This way these files can be piped to another command. Something like:

retest files origin/develop
=> 

spec/requests/api/v2/models/example1_spec.rb
spec/requests/api/v2/models/exmaple2_spec.rb
spec/models/post_spec.rb

• Maybe we only want to return the spec files for now?
• Should we return the changed files too?