3.0.0-alpha.1

• 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 (I hope).

Units are now explicitly defined in the map, instead of being magically converted in the background. We have a convert-units function you can use for making explicit adjustments:

$sizes: (
  'root': 16px, // will output px
  'text': 'root' ('convert-units': 'rem'), // will output rem
);