Save notification for later

[kpfefferle]

I would like some way to keep a notification in the list even after I've viewed it (rather than disappearing once I've viewed it like on github.com). This could be approached a number of ways:

1. Don't mark notifications as read with GitHub API until some action is taken (default to unread)
2. Allow "Mark as unread" on the client side at some point before persisting read notifications to GitHub
3. Maintain some sort of independent store of notifications that I want to keep reference to (most complicated)