[Shock] - Let users place theme files in a directory outside of source

[matthew-carroll]

Let users place a theme directory outside of source, so that a single theme directory can apply to multiple websites with a repository, or within a file system.

Simplest example:

my_website/
  source/
    _includes/
    index.md
  my_theme/
    _includes/
    about_us.md

A more complicated mono-repo example:

my_repo/
  website1/
  website2/
  my_theme/

Themes should support all content type, e.g., pages, assets, layouts, components.