Auto-generate font proofs

[alerque]

See e.g. this comment:

We could make some, but I’m left pondering whether it makes sense for every libre font repo to have it’s own set of idiosyncratic proofing documents rather than, say, a common script repo that generates proofing documents based on character coverage and OTL features?

It should be pretty easy to scan the coverage and features of font sources and generate useful proofs. The trick will be less in generating everything that could be covered and more in filtering the proofs down to just what is most useful/relevant to review.

Possibly leverage fontproof for this.