Use `man` for displaying the --help <foo> manpage

Fixes #368: --help does not clear the screen when done

The problem never was that it doesn't clear the screen, the problem was that
the man page didn't work as any other man page on the system. By using `man`
the behaviour is just like expected. If `man` clears the screen on your
machine, `hub --help <foo>` will now too.