Kebab-case public event names

[tumes]

I've submitted precious few PRs to other projects, so please excuse me if I'm unfamiliar with the process.

I was using your wonderful datepicker on a project when I noticed that the public event names don't adhere to Vue's guidelines and thus the Vue dev tools complain at you if you try to use them. I went ahead and modified them, plus retained camel-cased versions of the old events so it's not quite so jarring a change.

I imagine there are some non-public events that may also need updating, put I don't have a tremendous amount of time this afternoon, my apologies 🙇

[coveralls]

Coverage remained the same at 100.0% when pulling baff4bd on tumes:kebab-event-names into de0fc75 on charliekassel:master.