Consider adding 'custom hooks'

[vmadman]

Although Grits plans to continue to improve upon its own post-processing mechanisms, and also offers a fairly easy way for users to create plugins that have "hooking logic", it might be useful to another hooking layer that allows users to tell Grits to run external executables/commands in response to certain events.

For example, the user might add a hook that attaches to the afterCopy operation for files matching a glob (images/thumbnails/**/*.png) that runs an external command (such as generate-sprite-map {parentDir}).

Additional Notes:

• We should include as many hook events as possible
• Plugins should be able to extend hook events (e.g. triggering custom events) for their own purposes
• Issue Make Grits.js is FULLY asynchronous (via promises) #81 may or may not be required to make this work properly