Add paths to enable theming

[raffaelj]

fixes #2101
fixes #930

I added a simple, but dynamic system for paths to simplify theming, to enable child theming and to use shorter path names in renderFile shortcode. It is a concept from the Lime micro framework, which is the core of Cockpit CMS. I modified that concept to allow relative paths, which are used heavily in 11ty. Cockpit uses absolute paths instead. See also https://github.com/agentejo/cockpit/blob/next/lib/Lime/App.php#L348-L391

Usage:

+ _layouts
  + themes
    + base
      - default.njk
      - footer.njk
- .eleventy.js
- index.njk

.eleventy.js

eleventyConfig.path('views', './_layouts/themes/base');

index.njk

---
layout: "views:default.njk"
---
some content
{% renderFile "views:footer.njk" %}

See more usages in the test files.

Let me know, if I did some mistakes with contributing guidelines etc. - and I'm prototyping here. So there is some cleanup with commented code to do. I'm fine, if this pull request doesn't make it into the core, but I would love to see a similar concept. If there is no chance, or if it would take a lot of time, I would fork the RenderPlugin, so I can use the child theme behavior with rendered partials and some hacks to simplify the layout system.