Feature/riak cli handoff enable

This adds CLI commands to enable/disable handoffs:

riak-admin <enable | disable> <inbound | outbound | both> [--node <NODE>] [--all]

If handoffs are disabled, any currently active handoffs in the specified direction will be terminated.

If the riak-admin "set" command is used to disable handoffs in a given direction via the newly exposed handoff.disable_inbound or handoff.disable_outbound settings, corresponding handoffs will also be automatically terminated.

This PR also includes some additional unit testing in riak_core_handoff_manager.
This callback must return a `riak_cli_status:status()`. Use a `[riak_cli_status:text/1]` status instead of printing directly with `io:format`.

Yes, I realize this changed after you submitted this PR. Moving targets are hard.  :dash: