Implement "Favorite Hooks" feature

[denschub]

Some kinds of hooks with specific settings are used over and over again, and they have to be manually created every time. A list of "favorite", or even "frequently used" hooks would make work less repetitive.

See #15 for UI ideas.

[miketaylr]

This one feels a bit more of a stretch goal, so I didn't add the q1-2019 label. But if it turns out to be simple and we can get it done, that would be great. 🎉