rails - support command shortcuts

Recognize shortcut commands supported by rails. Don't provide the
shortcuts as completion possibilities since all but one of them is only
a single character, but if one is used allow further completion to be
done for the command's arguments as if the full command was entered.
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'.