allow overriding subtheme

[mahbutter]

This implements the feature requested in #3015

• Continues allowing user to specify a theme directory different from theme/
  • Combines certain files from theme/ if existing
    • Favicons (overrides entirely)
    • head.hbs (combines by the UTF-8 encoding)
    • highlight.js (overrides entirely)

This is useful when you're auto integrating an external theme, like Metro design, where you don't need to modify the external theme's files.