(#7683) Add a 'man' face and subcommand to Puppet.

This is the minimal wrapper, cloning a good deal of the logic from help, that
runs our face through the 'man' template and returns ronn-formatted Markdown.

This provides the crudest baseline possible for getting man-style output, but
lets us move forward to improve behaviour.

Reviewed-By: Nick Fagerlund <nick.fagerlund@puppetlabs.com>