Add virtual templates

[paulrobertlloyd]

Fixes #290.

One issue I’ve discovered; includes and layouts directory are relative to the input directory. If you use a relative path (i.e. ../layouts), Eleventy will look in that folder and ignore the virtual templates (and/or their template paths).

To do

• Update docs
• File bug on the above on the official Eleventy repo: Virtual Templates: more control over collisions between physical and virtual templates? 11ty/eleventy#3254 (comment)
• Test on Design History template – is it easier to override layouts provided by the plugin, and by how much

Possibly in a separate PR

• Move all plugin code into /lib
• Rename /lib to /src

[paulrobertlloyd]

With a virtual template provided by the official Eleventy RSS plugin, do we want to bundle that with this plugin, or just let users opt into generating RSS feeds via that plugin (perhaps only providing a pointer to the instructions for doing so?).

Otherwise we might just be adding in a layer of needless complexity and obscurity, and possibly require even more configuration options that we’d need to pass around, document and maintain. @frankieroberto