make the location of USAGE file configurable

[sinisterchipmunk]

Imagine this scenario:

Lots of generators:

lib/generators/foo/bar/bar_generator.rb
lib/generators/baz/bab/bab_generator.rb

... some of which reuse the same templates in different contexts. So they set a common source_root to lib/generators/templates.

Rails will look for a usage file in lib/generators/USAGE and will use the same usage file for both generators! Not necessarily a good thing!

This pull request exposes a usage_file class method on Generator::Base to allow explicitly setting the location of the usage file.

[josevalim]

If you want to change the location of the usage file, you can easily do: desc File.read("path/to/file"). Thanks for the pull request but I don't think we need to add more configurations on top of that.

[sinisterchipmunk]

Yes, but doing this and retaining the functionality from #3494 starts to get messy:

desc ERB.new(File.read("path/to/usage")).result(binding)

Also, this doesn't allow deferring of the ERB template processing until runtime. It must be evaluated at class definition, at which point the application may or may not have loaded. This has an effect if the generator is sensitive to the Rails application configuration.