Add convenience Ruby files regexes to DSL

[rafasoares]

Sometimes you may want to work on ruby files in the root folder or with an arbitrary structure (not necessarily in the lib/ folder).

This just adds those cases to the DSL, for convenience sake.

Example usage:

  require "guard/rspec/dsl"
  dsl = Guard::RSpec::Dsl.new(self)

  # This is similar to the current `ruby.lib_files`
  # Ruby files
  ruby = dsl.ruby
  dsl.watch_spec_files_for(ruby.all_ruby_files)
  # Or
  dsl.watch_spec_files_for(ruby.root_files) 

I looked for DSL specs, but didn't find any. If there are, please point me in their direction and I'll update them.