Add support for multiple commands

[tombruijn]

Add more commands to apps so we can use apps to test multiple things and in automated ways.

For example:

• test command to run Rails and JavaScript tests, for Regularly verify test setups #9 and debugging
• build command to run webpack build and test sourcemaps
• alternative to up that does install all dependencies but doesn't start any app. Convenient to develop integrations against if you need more control over what starts when.