Implement the core gen functionality

[etolstoy]

This task includes the implementation of this logic:

gen :template_name do
  set :some_option, '123'
  any_custom_action(parameter1)
end

Besides it, we should also handle the case with multiple gen's. We decided that their result will be written to a global array that will be automatically passed to save commands.

See 2.0 roadmap for references.