Add `depth` option

[camden]

See: https://git-scm.com/docs/git-clone#git-clone---depthltdepthgt

The functionality can be seen here in my fork... overall a pretty simple addition:

https://github.com/camden/node-git-clone-or-pull/blob/master/index.js#L71-L73
https://github.com/camden/node-git-clone-or-pull/blob/master/index.js#L123

[strugee]

FWIW to anyone who stumbles on this, I'm not going to spend time doing this myself but I would be happy to take a PR.