Pull in the list of always on features from the specification.

[garretrieger]

https://w3c.github.io/IFT/Overview.html#feature-tag-list

Utilize these when forming subsets for table keyed patching and for computing glyph based segmentations.

We have some existing code under ift/feature_registry which did this for the old patch subset spec, it should be possible to update this to work with the new format in the latest version of the spec.