This rewrite allows "long options" but unfortunately does not allow
merged options such as `-vn`.
Also improve the home-manager manual page, with this it should include
all sub-commands and arguments.
Finally, include the home-manager manual page in the generated HTML
documentation.
Unfortunately this duplicates some code from NixOS but it does allow
much more flexibility and, hopefully, stability in the Home Manager
documentation.
Fixes#254.
