Add xtask infrastructure to simplify local development work flow.

Created by: adamreichold

This should simplify running the programs without shell or batch scripts, e.g. running the harvester (and the indexer) and then the server should be as simple as

> cargo xtask harvester
> cargo xtask server

with reasonable defaults, e.g. using data and listening on 127.0.0.1:8081.

Furthermore, this as a default action to format, lint and test the code which can be used in edit-compile-test loops invoked as

> cargo xtask