Add autocmd with arbitrary data for autocommands

[thalesmello]

I wanted to use autocmd to trigger specific actions when GhostText was
activated (namely focus my Neovide instance), and creating an autocmd using au nvim_ghost_user_autocommands User * ! open -a iTerm was too broad, as it would
trigger for any User event defined by other plugins.

So I created a new autocmd that is then triggered with nvim_exec_autocmd, but
passing the url via the data callback property, only available in the
nvim_create_autocmd function call. It's notably more useful when writing
config files in Lua.