Support aliasing into $extensions

[drwpow]

Ex:

color: '{color.blue.500.$extensions.rgb}'

[drwpow]

So I’m not confident about this, but I believe this would be very hard to implement in the current version, because it’d not only involve some hacks to core; it’d also mean redoing a lot of work in every plugin. Instead, I believe the 2.0 release could make this much easier to pull off, and would allow for better handling of this at the core level.