custom extractors registration, add example convenience shell script for managing translations

Why not use an abc here with __call_ as abstract method? That will immediately result in a readable error when someone tries to make a derived class with `__call__` missing. See [this rest_toolkit](https://github.com/wichert/rest_toolkit/blob/master/src/rest_toolkit/abc.py) code for an example of using abc's for this purpose.