support caching check results

[lukpueh]

Checks will typically perform API requests. It would be great to cache them, for performance, or to not strain rate limits (especially during prototyping new checks, which might depend on results from existing checks).

Currently, all check results are written to a global context and JSON-dumped to stdout eventually. It would be easy to add an option for reading that data from a file, plus an option to skip checks, whose results are already present in that file.

This puts the onus of cache invalidation on the user, which should be fine for a start.