Allow plugins to subscribe to host events once

This adds the methods `onceHostEvent` and `onceAfterHostEvent` to `Plugin.Base`.  I figured that `once` and `onceAfter` deserved the same convenient shortcuts as `on` and `after`.

It looks like I also deleted some trailing spaces that were in the file.  Sorry for the diff noise.  I can redo the pull request if those spaces were important for some reason.