Create a file task for command-reference.md

This makes the Rakefile more make-like
Depend on rubygems checkout

This will make updating the command reference more repeatable.
Depend upon the current Rakefile

So editing the rakefile rebuilds the documentation
Silence updates of command-reference.md

When the command-reference.md is updated it needs to show the help to
initialize the options list.  Wrapping the call to show_help with a
silent UI prevents it from spamming help pages to the console.