Expand Milestone Types

If authors want to use milestones to mark progression or reflect in analytics, we need to track that information.

Track Word Counts (What would we call this Milestone type? Progression?)
Published could provide an opportunity to attach a purchase link and notify followers that it's ready to buy. Preorder might also be nice.

What other milestones would be nice, and what information would you want to track with it that could be quantified for you later?