Add a task to load defaults

[seenmyfate]

Now gems can implement a 'load:defaults' task in order to set any
defaults that may rely on other varibles set in stage configuration.

namespace :load do
  task :defaults do
    set :my_file, release_path.join('my_file')
  end
end

[kirs]

Great! I have to use these hooks in cap-rbenv and cap-rvm.