Improve play command's ability to locate bundle

[jcowman2]

Overview

The play command will have a better user experience if it has some tricks for finding a game bundle without the user having to type the whole thing out.

Description

Two improvements can be made to the play command:

1. The .regal.js file extension can be omitted. If there exists some-game.regal.js in the config directory, the command regal play some-game should load it automatically.
2. If no bundle path is specified when executing the play command and an output option is configured in regal.json (or equivalent), the CLI should attempt to play a bundle at that location. If it can't, an error should be thrown.