Add validation of command line and cookiecutter.json options

[seanh]

Cookiecutter supports a pre_gen_project.py hook that can be used to validate template options: https://cookiecutter.readthedocs.io/en/stable/advanced/hooks.html#example-validating-template-variables We should consider using this to validate our options: crashing if any unknown options are given or if a known option is given an invalid value.

If we're lucky this will also validate the contents of projects cookiecutter.json files when you run make template.

This would help to avoid issues like #129