Add a tip-of-the-day

[aeisenberg]

The goal behind this feature is better discoverability of existing features. Many users are unaware of the different features available in the extension and there is no single place where they can be reminded of it.

I'm not exactly sure of the best approach, but perhaps on startup we can cycle through brief descriptions and links to more info for some of our more interesting features.

Obviously, there needs to be an easy way to disable.