create a `document` method in DSL::Common to enable loading document from external file.

In my current project, I found it is annoying to have a very large string in my controller to describe an api, I want to keep my controller clean as much as possible, so I created this method, which will load a external markdown file as my description, this approach makes my controller much cleaner. my commit is cede4233ee7e51274a20efd200bf4ff75a40a33d, but I don't understand why I got 6 commits here(where the other 5 has been merged, so I think it's ok)