Support for Italic axis on API?

[cssobral2013]

So, it'll avoid the need to make independent italics.

[m4rc1e]

The api is fine. The issue is the ital and slnt axes lack decent browser support https://arrowtype.github.io/vf-slnt-test/

[davelab6]

Families composed of a single file with a slnt axis, we can enable that axis in the API, but (for now) it will not show in fonts.google.com

Families composed of a single file with ital axis, we can not enable that axis in the API, because a single @font-face will not work with font-style: italic yet, as explained on https://arrowtype.github.io/vf-slnt-test. Therefore we recompose the family with italic as an independent file, so that it is delivered with 2 @font-faces, and font-style works as expected.