lua: Add assert
lua: Add assert

Standard lua assert message.

msg is optional, so hide the ", " if it's omitted.

honza/vim-snippets,phatograph/vim-snippets,kevinkirkup/vim-snippets,craigp/vim-snippets,phatograph/vim-snippets,kevinkirkup/vim-snippets,adam-e-trepanier/vim-snippets,honza/vim-snippets,adam-e-trepanier/vim-snippets,craigp/vim-snippets