add simple lua convenience wrapper

[willnorris]

Hey Josh, how would you feel about a very small lua wrapper around the public functions in onedark.vim? I just did this to rewrite my onedark customizations in pure lua, and I'm kinda loving it!

https://github.com/willnorris/dotfiles/commit/e512964f0a65ff29a35467360665ddd00e5be909

commit attached here if you're open to it.