Adding incremental transfer of VF axis spaces to the spec

I took a pass at adding the ability to incrementally transfer parts of the
axis space in a variable font to the spec:
https://github.com/w3c/PFE/pull/11

It works similar to codepoints where the client now has the ability to
declare the axis space they currently have and any additional axis space
they'd like. By default if the client doesn't specify anything it will
result in the full axis space being included in the subset. This makes the
feature optional. If a client doesn't care about getting a narrower axis
space then they can just  omit the new fields in their request.