Implement an optional promise based interface

[sam3d]

I don't know how possible this would be, because I can't remember if anything is returned after running something like serf.members(cb). it would be great to have the option of doing both of the following as if nothing is returned it wouldn't break the existing API (obviously this wouldn't work for the streams):

Standard

serf.members((err, members) => {
    console.log(members);
});

With promises method

serf.members()
.then(members => {
    console.log(members);
})
.catch(err => {
    throw err;
});

With await/async

let members = await serf.members();

Is this something that would relatively easy to implement or is it architecturally a bit of a pain?

[zbjornson]

Nothing is returned from the command methods except for the three streaming commands (stream, monitor and query). Looks pretty easy to do.