builder: Ability to set font version in config file

[m4rc1e]

It would be nice to have a field in the config file which sets the version number for all fonts. Imo it should take precedence over all other forms of setting the version number.

cc @simoncozens

[simoncozens]

Definitely! In the source font or in the binary?

[m4rc1e]

Atm, I'm thinking just the binaries. Personally, I find the builder is simply adding features that don't exist in the source formats yet so we shouldn't tamper with existing sources.