[refactor] split archetype into multiple extensions

This address #67

Archetype has now been split into smaller projects
- the core will provide the most widely used functions/mixins with much opinion on your generated CSS
- things that are more specialized, opinionated, or heavy-handed have been moved to extensions, these extensions currently include:
  - `archetype-base` _this will likely be split into multiple extensions_
  - `archetype-grid`
  - `archetype-theme`
- the goal of this split is to provide consumers with the most useful and non-intrusive features upfront while still offering (via extension) the robustness we've already built