Revise man page (and its generation)

- Add documentation for the `--[no]group` and `-v --invert-match`
  options.
- Rephrase the descriptions of some options to better distinguish
  between those that actually operate ON filenames (e.g. -g, -G),
  and those that output filenames, but scrutinize their contents
  (e.g. -l, -L).
- Hyphenate "case-[in]sensitively".
- Insert line breaks between each option in the "OPTIONS" section.
  This leads to output, particularly indentation, more consistent with
  "regular" man pages, while at the same time requiring less
  preprocessing work.
- Revise generate_man.sh to deal with the only formatting issue
  still remaining: null characters in Markdown need an extra layer of
  escaping for ronn to handle them correctly.
Tweak the `--vimgrep` documentation slightly
Merge pull request #517 from hivehand/master

Revise man page (and its generation)