Documentation: options.config

Based on the [compiler source](https://github.com/strongloop/loopback-boot/blob/master/lib/compiler.js#L31), an `options.config` value can be supplied to the `boot` initializer as an alternative to maintaining a `server/config.json` file. The [boot documentation](http://apidocs.strongloop.com/loopback-boot/#bootapp-options) should be updated to reflect that this option is available.
