Add --mode derivative,inplace option

[yarikoptic]

Assuming that current default is "derivative" where defaced images are placed into some new folder.

Most often though, I think, users would want to deface BIDS dataset "inplace" , gain _defacemask file(s) and may be move original into sourcedata/ .

Check e.g. what bidsonym does: https://github.com/PeerHerholz/BIDSonym/blob/23317f6b9fef4b4275a41d1a6acf998c035b893b/docs/source/outputs.rst#L17