Keyword lookup changes

`man` lookups don't work well in gvim (or vim, really).  In gvim, use
`man -H` for the html version if available (requires groff).  If editing
a vim file, use `:help` instead.

Also source man.vim, which can be called with `<Leader>K`.  I might
remap this to `K` if I can find a way to keep keywordprg around.