Clipboard monitor

This PR includes a bunch of changes that add a stack of extra functionality to `extapi` for monitoring the target machine's clipboard. A monitor thread can be created and run behind the scenes, and while running it captures the clipboard data as it appears on the clipboard.

For details of how it all works, check out the sample run on the [MSF side of this PR](https://github.com/rapid7/metasploit-framework/pull/2930).

Existing clipboard functionality still works as expected, this is just an enhancement to it so that harvesting things from the clipboard as they appear is possible.