Support minimalist installations with no config.ru or Rakefile (or anything other than Gemfile really)

[jaredcwhite]

I thought #941 would be harder than this…maybe we can sneak it in before the final 2.0 release!

With these changes, your project folder could look like this:

* Gemfile
* src
  * index.erb

And then you could run bundle exec bridgetown start and it'd actually work (built files in output as expected). 🙌

Todo

• Do something helpful if the src folder doesn't even exist. Maybe autogenerate an index.erb file with a welcome message.

Fixes #916