rail - Add completion for scaffold generator

Implement completion for rails scaffold generator, most of the arguments
are shared with the model generator so use a common function to generate
those.
rails - Support uniq indexes in model generator

Support creating unique indexes with model/scaffold generators for rails
by completing ':unique' in addition to ':index'.
Merge branch 'rails' into run

* rails:
  rails - Support uniq indexes in model generator
  rail - Add completion for scaffold generator