First-class functions & negative calc

• BREAKING: Remove all non-essential default sizes
• BUGFIX: Add convert-units to local (pre-captured) functions
• NEW: negative function can be used with calc() values,
  returning calc(-1 * (<value>))
• NEW: $functions map allows you to name/capture first-class functions
  in the latest versions of Sass, and call them by name in the $sizes map
• NEW: add-sizes & add-ratios functions can be used
  to merge partial maps into the global $sizes/$ratios maps