Rewrite in Lua

[aileot]

The main reasons to rewrite this plugin in Lua are as follows:

1. Extensible
   Users can set functions to foldpeek's option variables because Lua treats functions as a first class object.
2. Fast
   Lua is faster than Vimscript as is well known.

However, Lua plugin would be only available in neovim because I don't want to know if_lua interface.

[aileot]

6604983 suggests any option variables could accept Funcref so that the reason 1 above (extensibility) would no longer encourage rewriting this plugin in Lua.

[aileot]

Still, this plugin has performance issue.