Improved flexibility with $do and $source args

• Core:
  • NEW: ratio() function accepts $source argument,
    for passing in a custom source map
  • NEW: Improved error-handling and messages
  • NEW: Added str-trim and str-split aliases
    in addition to the existing trim and split
• Plugin: Animate
  • NEW: ease(), time(), and change() functions all
    accept $source argument,
    for passing in a custom source map
• Plugin: Layout
  • NEW: break() function accepts $source argument,
    for passing in a custom source map
  • NEW: break() function accepts $scale boolean,
    to turn off access to the scale-plugin $sizes map
  • NEW: fluid-ratio() function and mixin both accept $source argument,
    for passing in a custom source map
• Plugin: Scale
  • BREAKING: negative() function and square() mixin
    both accept $do argument for on-the-fly adjustments --
    replacing the old $units… variable argument.
    Non-map $do values are converted to
    ('convert-units': $do) before processing,
    to provide a shortcut for unit-conversions.
    negative('root', 'cm') will continue to work,
    but negative('root', 'em', 10px) should be changed to
    negative('root', 'em' 10px) (with all unit args in a single list)
  • NEW: size()/negative() functions and square() mixin
    accept $source argument, for passing in a custom source map
• Plugin: Type
  • NEW: font()/font-family() functions
    and font-family()/font-face() mixins
    accept $source argument,
    for passing in a custom font-source map