optional Promise API

[jneuendorf]

What do you think about supporting async and await (aka. a Promise-based API)? I think it would be greate since packages relying on fs-plus (e.g. atom/season) could easily use the cool ES7 async/await as well.
There is another fs extension node-fs-extra that returns Promises if no callback is provided (which I really like). Of course,node-fs-extra carries along some extra methods but those should be able to be ignored easily from what's exported by fs-plus (in case you even care).

Anyway node-fs-extra can be used as a drop in replacement for fs. So it'd be really easy for you to also support the Promise API by using node-fs-extra as a dependency in your package.json instead of the native fs module.

In case you're interested I'd be willing to create a PR.