Re-use cookiecutter values of containing project

[jhermann]

• Look for a contained cookiecutter.json or [project.d/]cookiecutter.rc
• And/or load values by introspecting the project itself, e.g. python setup.py --url

[jhermann]

Doing this needs new hook types pre/post_config_prompt called in prompt.py. These hooks must emit JSON (and nothing else) on stdout; logging can go to stderr.

The parsed JSON is merged (dict.update) into the existing context. This allows, among other things, for computed defaults.

[jhermann]

See also cookiecutter/cookiecutter#364

[jhermann]

Can be done similar to https://github.com/Springerle/py-generic-project#reapplying-to-an-existing-project, so now a docs issue for the README.