Vim: add mapping to hopefully fix most syntax highlighting issues

I'll occasionally land in a file and the syntax highlighting will be
wrong, requiring me to jump upwards until Vim finds a place where it can
meaninfully start syntax highlighting (I usually do "gg" to jump to the
top of the file then "C-d" a few times to get back to where I was
editing, as a straight jump [ie. with "``"] often doesn't fix the
highlighting).

Rather than trying to tweak my sync settings on a per-language basis,
let's just add a mapping to hopefully brute-force the issue whenever it
crops up.

Idea stolen from:

  http://vim.wikia.com/wiki/Fix_syntax_highlighting
Vim: make mapping comments more consistent