serve: Refactoring and fixes to be able to serve taxonomy pages. * Page sources' `findPagePath` is renamed to `findPageFactory`, so that it also returns source metadata. * Page refs now store possible hits more properly, and use the previous point to also store metadata. As a result, they can also return a proper factory. * Implement `findPageFactory` correctly in all built-in sources. * When the Chef server matches a taxonomy page, get the source metadata from the page ref in order to make a more proper page. * Make the `getRoute(s)` functions explicitely say if they want taxonomy routes or not.

---
in:
    posts/2010-01-01_post1.md: 'post one'
    pages/about.md: 'URL: {{page.url}}'
    pages/_index.md: 'something'
out:
    '2010':
        '01':
            '01':
                post1.html: 'post one'
    about.html: 'URL: /about.html'
    index.html: 'something'
---
config:
    site:
        root: /whatever
in:
    posts/2010-01-01_post1.md: 'post one'
    pages/about.md: 'URL: {{page.url}}'
    pages/_index.md: 'something'
out:
    '2010':
        '01':
            '01':
                post1.html: 'post one'
    about.html: 'URL: /whatever/about.html'
    index.html: 'something'