[Shock][CLI] - Themes should be pluggable

[matthew-carroll]

I've been working on a variety of websites, e.g., Super Editor, Lucid.

Having theme templates are nice from a starting point. However, one thing I'm noticing, is that I'm adjusting the Docs theme for a site like Super Editor, and then I want the same adjustments in Lucid. With the current approach, replicating theme changes requires copying and pasting all such changes across multiple sites. This is very error prone.

There should be three convenient ways to work with styles:

1. Use layouts, components, styles, JS, and assets from a remote source.
2. Use layouts, components, styles, JS, and assets from a sub-directory of the current project.
3. Use layouts, components, styles, JS, and assets from arbitrary locations within a project, similar to the current setup, but also allowing for any of these items to be defined next to content files, e.g., Markdown files.

With this adjustment, FBH can share one or more central themes, allowing us to keep our websites consistent, and improving them en masse.

This adjustment also makes it easy to blow away an entire existing theme and replace it with another theme, without needing to carefully overwrite specific files.

Finally, for truly per-project adjustments, its nice to have layouts, components, and styles near where they're needed, rather than forcing everything into a top level directory.