Create full extension library support

[JonHMChan]

Extensions should be:

• JavaScript classes
• Private attributes that describe the library, modifiable at construction
• Value methods that produce computed values for properties (shorthand for rgba(), modular font-sizes, etc.)
• Mixins that produce rulesets to be passed as mixins. Methods are prefixed with _
• A .transform that manipulates the style tree before it is rendered by Descartes. Should take in the style tree as an argument and return a new style tree. This allows for custom rules, rule prefixing, etc.

I've had an initial attempt with Plato.js - you can see that within the src for what this might look like.