treat *.md as markdown

I'll bet there's a better way to do this.

.vimrc

@@ -136,6 +136,7 @@ let mapleader = ","
 
 " treat p1k3 entries as HTML. lazy, but unlikely to hit many false positives:
 au BufReadPost,BufNewFile */p1k3/*[0123456789]* call BPB_PikeHighlight()
+au BufReadPost,BufNewFile *.md set filetype=markdown
 
 " the ! means these can be freely redeclared - makes it easier to source
 " ~/.vimrc after changes