Make fig run COMMAND parameter optional.

The new behaviour:

Same as previously:

    fig run SERVICE COMMAND

If there's a `command` defined for the service in the fig.yml, runs `command` (i.e. basically behaves like `fig up`, but with a TTY and `--rm` options:

    fig run SERVICE

If there's no `command` defined in the fig.yml, acts like the native client and runs `CMD` from the Dockerfile:

    fig run SERVICE

This cuts down my keystrokes considerably, as I'm almost exclusively using `fig run` to launch into dev environments in a console :)