v6.0.0

• [BREAKING] Default width and height can now be set by declaring defaults
  for object destructuring, instead of having custom options for them.
  This means the following options were removed: defaultWidth, defaultHeight,
  useDefaults.
• [BREAKING] Due to the above, the default width and height will be
  undefined instead of 1.
• Docs were updated to reflect the above changes.
• Added an onResize handler that can report size changes instead of the default
  object return. (This also helps implementing a more performant throttle /
  debounce solution.)