feat: Plugins that can add content to the site (components, layouts, etc.)

[jaredcwhite]

This is a "mega" issue to track the ability for Bridgetown plugins to supply a manifest that can be consumed and allow files to be added to the site build from the plugin folders same as if they were in the main site folder. This would include components (aka {% render "cool_plugin/widget" %}), layouts, pages, static files, and data. A certain kind of plugin utilizing these features could be called a "theme", but there are plenty of other interesting use cases.

I already have a bunch of this working locally in a proof-of-concept branch, which is pretty neat, but I think some of the usability issues need to be thought through carefully.

• What if you want to override one of the plugin's files? I don't think "fish around in a gem repo and copy the right file" is a good answer. Maybe we need some extra CLI commands.
• What if multiple plugins try writing to the same url/data/etc. location? Should the first one win? The last one? Issue a warning?
• What if the plugin wants to adjust what it provides based on a user's configuration? For that matter, how do we handle configuration? Dump everything in bridgetown.config.yml? Use separate data files?
• What if a plugin "goes" rogue and messes up other parts of the site? How would the user know how to troubleshoot the problem? At the very least I think we'll need more debug logging that can get switched on to know exactly which plugin is doing what at which time.
• What if plugin A wants to include plugin B as a dependency? (aka "child themes", or multiple plugin "monorepos")

[jaredcwhite]

Just making a note here that PR #41 resolves all of the questions I itemized above with the exception of "What if multiple plugins try writing to the same url/data/etc. location? Should the first one win?" (I'll have to go look up which one wins, actually!) and "need more debug logging". Things to address in followup enhancements I think.