Explicit unit-handling, and get-function support

• BREAKING: Don't convert units unless specifically requested.
  This allows you to define preferred units per-item
  in the configuration map.
• BREAKING: Remove $default-units setting. See above.
• Allow arbitrary adjustment functions
  in the $sizes map,
  e.g. 'my-size': 24px ('add': 12px),
  where add is an available function
  that will accept 24px, 12px as arguments.
• Support first-class functions in Sass 3.5.