Project-specific commands

[AlexWayfer]

Is your feature request related to a problem? Please describe.

I have a command for this plugin: bundle exec rubocop

But some projects have not rubocop in their bundle. OK, I can add it, and also here is #367 .

And now I've faced with project without rubocop at all, but with standard gem, which internally uses RuboCop.

Describe the solution you'd like

Can we somehow define commands per project? As exceptions.

Describe alternatives you've considered

I didn't find an Atom plugin for standard gem.

Additional context

rubyapi/rubyapi#383 (comment)

[vzamanillo]

#411