Add `enabled` flag to config

Allow Yokozuna to be enabled/disable based on the `enabled` config.
If set to `true` then Yokozuna will be started, `false` and it will
not.

* When disabled, the `yz_sup` process is still started but it will
  have no children.  Furthermore the webmachine resource will not be
  registered and the yokozuna service will not be marked up.

* I think you could enable Yokozuna on-the-fly, i.e. without
  restarting the VM, by changing the enabled flag to true and then
  running `application:stop` followed by `application:start`.