Support callbacks

(https://github.com/yeoman/generator/pull/262)

Added in support a check for an urgent response from the request for update information.

This could use cleaning up to not use config store at all if asking for a response immediately, maybe?

Currently, if a { callback: function {} } is passed in at instantiation, `this.sync` is flipped to true, which later forces an inline call to `this.checkNpm` as opposed to forking.