add simple lua convenience wrapper

This allows for using more idiomatic calling styles in lua.

lua/onedark/init.lua

@@ -0,0 +1,5 @@
+return {
+  get_colors = vim.fn["onedark#GetColors"],
+  set_highlight = vim.fn["onedark#set_highlight"],
+  extend_highlight = vim.fn["onedark#set_highlight"],
+}