Add more data for entity components (#91)

[thibaudcolas]

Fixes #91, in a non-breaking way.

Entity components now have access to the following data under props['entity']:

props['entity'] = {
    'type': entity_details['type'],
    'mutability': entity_details['mutability'] if 'mutability' in entity_details else None,
    'block': block,
    'blocks': blocks,
    'entity_range': {
        'key': self.completed_entity,
    },
}

I’ve also documented the shortcomings of the current implementation in the Troubleshooting docs. Addressing this would require breaking changes so I think it’s relatively unlikely to happen.

---

• Stay on point and keep it small so it can be easily reviewed. For example, try to apply any general refactoring separately outside of the PR.
• Consider adding unit tests, especially for bug fixes. If you don't, tell us why.
• All new and existing tests pass, with 100% test coverage (make test-coverage)
• Linting passes (make lint)
• Consider updating documentation. If you don't, tell us why.
• List the environments / platforms in which you tested your changes. make test-ci