core: Start of the big refactor for PieCrust 3.0. * Everything is a `ContentSource`, including assets directories. * Most content sources are subclasses of the base file-system source. * A source is processed by a "pipeline", and there are 2 built-in pipelines, one for assets and one for pages. The asset pipeline is vaguely functional, but the page pipeline is completely broken right now. * Rewrite the baking process as just running appropriate pipelines on each content item. This should allow for better parallelization.

---
url: /foo.html
theme_config:
    site:
        title: "Some Test Theme"
in:
    pages/foo.md: "This is: {{site.title}}"
out: "This is: Some Test Theme"
---
url: /foo.html
theme_config:
    site:
        title: "Some Test Theme"
in:
    pages/foo.md: "This is: {{foo}}"
    configs/theme_preview.yml: "foo: bar"
out: "This is: bar"