add recommended formatter setup for lua and nvim lua plugin template

[matu3ba]

1. https://github.com/Koihik/LuaFormatter https://github.com/JohnnyMorganz/StyLua looks best, since it is simple.

2. For everything plugin development related or to simplify the guide, I would recommend linking to https://github.com/nvim-lua/nvim-lua-plugin-template and to plenary lua ("inofficial neovim luajit libstd").