update api to match existing vim apis.

[prabirshrestha]

Would it be possible to update the api to use ids instead of object oriented style so it matches with vim? Would be awesome if you proposed the api at https://groups.google.com/d/msg/vim_dev/oubHkRbdQW0/TZYUUQftAgAJ

let watch_id= filewatcher#watch(".", {x,y->execute('echo y',0)})
call filewatch#stop_watch(watch_id)
let watch_status = filewatch#status(watch_id)

Since watch_id is a number I can serialize and deserialize this across jobs or a neovim remote plugin.

[mattn]

Could you please let me know the description of watch_status?