Feature Request: Return indices of elements from merge

Your package is great.

Using intersect with indices saved me a lot of time and headache.

It would be great to have indices on the merge method as well. This is what I currently do:

data = np.concatenate((a, b))
index = np.argsort(data, kind="mergesort")

Since my arrays a and b are sorted, it naturally comes to me that adding indices to merge would avoid having to mergesort my array.

Thanks in advance.