Start clipboard monitor functionality

Added the basics of the clipboard monitor functionality with usage
messages and stuff like that. Lots more to do.
Merge branch 'upstream/master' into clipboard_monitor

Conflicts:
	lib/rex/post/meterpreter/ui/console/command_dispatcher/extapi/clipboard.rb
More work on the clipboard monitor
More tweaking of the clipboard monitor with dump/purge
Small tidy of code
Final fixes before the monitor PR
Merge branch 'upstream/master' into clipboard_monitor

Conflicts:
	lib/rex/post/meterpreter/extensions/extapi/tlv.rb
Merge remote-tracking branch 'upstream/pr/2930'