consolidate: allow using existing manifest as a cache

Consolidation already takes quite a while, and it's generally unlikely that existing tags will have changed labels or a changed structure. The default behaviour should be to use the existing manifest file as a cache, and only fetch data for _new_ tags, with the _option_ to instead do a clean/full refresh and generate the entire file from scratch.

Something like a `--cache` command-line argument could be useful, where
- if unspecified it uses the latest online manifest
- if "none" it does a full refresh
- if given a URL it tries to fetch it and use it, erroring out on failure to fetch
- if given a local filename it tries to read it and use it, erroring out on failure to read

This should save a bunch of consolidation time, and also reduce unnecessary API usage.