Add make targets for single-format releases

[alerque]

The current strategy of generating a single release bundle with all target formats (TTF, OTF, WOFF, WOFF2 for both static and variable for a potential of 8 formats) is generally useful, but it's not what all projects need.

As a follow up to #88, I suggest we add optional targets that would generate release packaging filtered to specific output format(s).