feat: support "minimalist" Bridgetown projects

[jaredcwhite]

I thought there was an issue like this floating around, but I couldn't find it exactly so here's a new shiny one.

From time to time, I noodle around with experimental project folders which have the fewest amount of config files possible. The idea is that a "minimalist" install of Bridgetown could build/service a folder with virtually no config files…ideally, only Gemfile! (so config.ru optional, as well as a Node-less environment)

[jaredcwhite]

Hmm, is there a bug with start --source=foldername if using something other than the default src?

[jaredcwhite]

Yes there was! I think we may have a remaining timing issue with when certain paths get set across defaults, CLI options, and the initializers file, but we should file those separately if so.