Use canonical <Plug>-mappings.

By invoking an autoload function on `VimEnter`, you're not getting the full benefits of them. Also, `<Plug>`-mappings (to me at least) are the canonical way of allowing remappings.

I've made these changes to your excellent work. Let me know whether you're interested in future pull requests (there won't be many, as I'm not actively working on the plugin). I'll be watching your repo for sure.