Enable snippet plugins which do not recognize 'mkd' filetype

When I installed `vim-markdown` my snippets stopped working. It was due to the mkd file type. 

This solution was copied from:
https://github.com/tpope/vim-rails/blob/master/autoload/rails.vim

Alternatively this plugin could use `markdown` file type but I guess there was some other reason not to do it in the first place.