Configurable root path

[tomasc]

This PR allows for configuration of the root path, where Scenic looks up its views:

Scenic.configure do |config|
  config.root_path = MyEngine.root
end

The default value stays Rails.root.

This is a follow up to #237 (comment) and addresses #261.

[derekprior]

@tomasc how would this work in cases where both the engine and the app itself want to use Scenic?

[trevorrjohn]

I would think you need two config files for that, one in the engine and one at the app level. Or you could add a env var or argument to override the "default" path.

[derekprior]

The configuration is currently a singleton, so I don't think that would work

[derekprior]

We have a number of proposed changes (see: view-paths ) that cover this same functionality. I think #237 is closest to something we would consider merging in that covers multiple use cases. Swapping one static path for another would solve some problems but create others.