add intermediate flag to include intermediate parent keys in the result

[nabiltntn]

When using the new intermediate flag to true, the result will include intermediate parent keys.
Exemple

var obj = {a : {b : {c : 1}}};
deepKeys(obj, true);
//=> ['a', 'a.b', 'a.b.c']

[a8m]

Thanks, @nabiltntn
I released a new version and added you as a collaborator