Aliasing font mixins

See https://github.com/Financial-Times/ft-origami/pull/150

I've more or less copied the mechanism from o-colors to enable modules to request fonts based on use cases - if a use case doesn't require a font face declaration then no css is output. If 2 use cases require the same font face o-fonts already takes care of preventing duplication.
@triblondon @dansearle-ft