(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.
Another alternative would be to do this in the `cli/apps/irb`:

```bash
 --config ${CONFIG} -- -- "$@"
```

If we do it that way, though, users of the lein alias would have to still enter "lein irb --config ... -- --" in front of any args to irb that they want to specify.

By not including the "--" in the clj file, we would allow for the user to specify args that can be honored by JRuby main vs. ones for irb, e.g., -- <jruby_main_args> -- <irb_args>.  @nwolfe were debating the value of doing that.  If users were to desire passing any arguments at all and the ones being passed were exclusively for irb, forcing the user to type the one extra "--" seems undesirable.