(SERVER-204) Subcommands for 'ruby' and 'irb'

Working implementations for ruby and irb to go along with the gem
subcommand. Acceptance tests need to be written, as well as docs.
In some cases this is necessary in order for the user to be able to pass in additional CLI args to ruby/irb; @fpringvaldsen hit this with the gem command originally, I thought we were basically following the pattern that he had established?