Improve support for context extensions

[simleo]

#53 added initial support for context extensions, but this is currently limited to updating the context with testing namespace terms when a testing-related entity is added to a crate via specific methods such as add_test_suite. The main thing that remains to be covered is preserving any context extensions when a crate is read, so they don't get lost in the read-write round trip. Also, when adding an entity to a crate, we should probably add only the relevant extension to the context, rather than dumping the whole shebang like we're now doing for testing-related entities.

See ResearchObject/ro-crate-ruby#17.