Add header mapping for markdown

I like the big underlines for headers and do them often enough to make a
map instead.

Re-uses map for top-level header comments since those aren't meaningful
in markdown.
Default to vertical diffs

Fugitive now assumes horizontal unless you set diffopt to use vertical
diff splits. I think I usually want this behavior (although I'd rather
it be smart based on how much space is available -- oh well).
Make gvim work in MINGW

Every time mingw in Github for Windows updates (which it does
automatically), it cleans out the old directory so I can't put stuff
there. It's easier to just add vim to the path.
Include my usrlocalbin in path

Instead of trying to do fancy things where I add a link for
/usr/local/bin, just include that directory in my path.