Treat .md files as Markdown

Fixes #418.

![test.md fixed](https://cloud.githubusercontent.com/assets/205659/4004805/de60fbd4-2986-11e4-95a6-de0aa371ef1c.png)

[filetype](http://vimdoc.sourceforge.net/htmldoc/options.html#%3asetfiletype)
is set implicitly, without using setfiletype, because `*.md` is already associated with Modula-2 files and `setfiletype` doesn't allow `filetype` to be set twice.

[setlocal](http://vimdoc.sourceforge.net/htmldoc/options.html#:setlocal) is used to apply a new `filetype` value for each Markdown file as it is being added to the buffer.