Support additional event types

We only have Create, Remove, Rename, Write, and Chmod now, but many watchers support additional events. This has come up a few times:

• #24 - Extend, Link, Revoke
• #297 - Unmount
• #346 - Read
• #504 - Open, Close
• Probably some others?

Instead of creating a bunch of issues for every possible event, track them here.

All of this depends on #7; we don't want to spam people with e.g. Open and Close events when they're not interested in them (which is most of the time), and some events aren't supported on all platforms.