feat: add support to promise

[picheli20]

Solve #6

[jaz303]

I'm not willing to break support for the callback style as this project has a large number of dependents.

It's trivially possible to automatically convert the callback async style into a promise-based equivalent (e.g. via es6-promisify) so I would question the need for this.

I wouldn't be against providing this wrapper in the main package, e.g. require('git-clone/promise'). How does this sound?

[picheli20]

yeah, sounds good as well. I'll update my PR

[Sceat]

@jaz303 could this be merged and published on npm ? thanks

[MichaelDimmitt]

👍