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: /pr%C3%A9sentation.html
in:
    pages/présentation.md: 'PAGE URL: {{page.url}}'
out: 'PAGE URL: /pr%C3%A9sentation.html'
---
url: /2010/01/01/d%C3%A9j%C3%A0-des-accents.html
in:
    posts/2010-01-01_déjà-des-accents.md: 'POST URL: {{page.url}}'
out: 'POST URL: /2010/01/01/d%C3%A9j%C3%A0-des-accents.html'
---
url: /special/%D0%AD%D1%82%D0%BE%20%D1%82%D1%8D%D0%B3.html
in:
    pages/special/Это тэг.md: 'PAGE URL: {{page.url}}'
out: 'PAGE URL: /special/%D0%AD%D1%82%D0%BE%20%D1%82%D1%8D%D0%B3.html'