Feature request: cache downloaded contexts/vocabularies between runs

[elichad]

When using the validator on a slow internet connection, it can take a long time to download the necessary context (and anything else that the package downloads). When running repeatedly while improving a crate, running tests, etc. this can slow down the development process considerably.

Caching downloaded files would help keep the validator speedy on slower connections. In most cases I assume the cache could probably be kept for some time as contexts/vocabularies are quite stable (and you could add a --no-cache option to force re-downloading if needed).