Initial sketch
Cleanup; GSUB/GPOS mostly done

The remaining formats are not in wide use apparently!  Will implement
when I encounter them.
Implement GDEF
Implement remaining GSUB/GPOS [Chain]Context formats

All implemented now.
Start implementing pruning
Implement pruning

Seems to be working!
Minor
Implement LookupList, FeatureList, ScriptList backwards pruning
Add --verbose
Add --xml / --verbose
Implement PairPos pruning

Now that's the real deal...
Implement mark positioning class matrix pruning
Start adding other tables
Implement 'kern' table
Minor
"Implement" tables that don't need subsetting
Drop unneeded tables
Subset GlyphOrder
Don't load tables that don't need subsetting
Implement vmtx, hdmx, VDMX