Support regenerating for existing versions

[ES-Alexander]

Current output generation is efficiently capped to ArduPilot tags that are newer than the latest commit in this repository.

That's nice to avoid unnecessary extra processing time, but it can also be useful to occasionally regenerate files for older versions, in case there have been changes in parameters that are reported as supported, and to include new MAVLink messages in the explicitly unsupported lists (e.g. as in #10).

It would be useful to add a command-line argument to the run_parsers script that allows overriding the "latest" date, and potentially running that via CI at a lower frequency (e.g. once a month).