Allow pages and layout directory pages to be configurable

[Undistraction]

At present there is no way to change the location of the pages and layout directories in a project. This makes organising a project's src dir harder, because all JavaScript files can't be separated into their own directory and you end up with:

- pages
- layout
- styles
- javascript
- images
- fonts
- content
- etc…

By making the paths to these directories configurable they could be relocated to the javascript directory, making the src directory cleaner.

- styles
- javascript
- images
- fonts
- content
- etc…

Please note I am absolutely in favour of convention over configuration, but I feel that in this instance forcing directories of React components into the top level of the src directory isn't a good convention. A cleaner structure makes the site easier to use, especially by people with less experience in development (for example when handing off to a client who will be publishing their own content via the site).

See @KyleAMathews response to my Stack Overflow Question:

No it doesn't. Also fwiw, only pages and layouts are special directories. You can move templates and components to a js directory.