Syntax highlighting for code blocks

Based on the code from [vim-notes](http://peterodding.com/code/vim/notes/), adds syntax highlighting for GFM code blocks. E.g.

    ```vim
    let g:foo = 1
    ```

is shown as syntax highlighted with the vim syntax highlighting.

This introduces a `plugin/mkd.vim` file. Putting this code in `ftplugin` meant that it didn't refresh the syntax on `BufReadPost`. 

As per `contributing.md`:
* It can be tested with `test\syntax.md`, you'll see the ruby block is highlighted.
* `readme.md` hasn't been updated, as it doesn't require any configuration