The current implementation of the help function sorts the command lexicographically. A request for enhancement (#361) has been open for a while and it seems that people are resorting to hacks to change it.

This PR adds a class method called sort_commands!. The default implementation maintains the lexicographic ordering but the method can be easily overridden by the subclass to obtain any sort of ordering.

Here is a quick example that would place the help command first

class MyScript < Thor
  ...
  def self.sort_commands!(list)
    list.sort_by! do |a,b|
      a[0] == :help ? -1 : a[0] <=> b[0]
    end
  end
end