better post commit hook enabling/disabling

* if --enabling when an existing hook is present
  * removes any existing lolcommit hook lines
  * checks for a good shebang line (and aborts if not found)

* --enable works as before if no post-commit is present

* if --disabling
  * when existing hook is present, removes lolcommit lines only
  * warns if lolcommits might still be present (in old style)
  * info message if no post-commit file found at all
Merge pull request #173 from mroth/better-hook-enable-disabling

better post commit hook enabling/disabling