Add ability to extend a theme's or module's namespace

A simpler solution to our "keep our templates and component libraries separate" is simply to allow a separate folder from /templates for the component libraries while sharing the Twig namespace.

Twig allows you to specify multiple paths for a Twig namespace. Drupal core only provides the /templates path for a theme's Twig namespace. e.g. bartik/templates is the path for the @bartik Twig namespace.

We could allow theme's to define additional paths for the theme's namespace.

template-paths:
  - components

would mean you can place your Twig templates in /templates or in /components directories.

Alternately, we extend the existing .info syntax:

component-libraries:
  THEMENAME:
    paths:
      - templates
      - components

[edit: we are going with the alternate as it is more consistent.]