new options:
  * options.outputOffset
  * options.ambiguousEastAsianCharWidth
  * options.diff

changes:
  * make string diff function pluggable
  * outputOffset option to configure number of spaces inserted at the left
  * set outputOffset default to 2
  * make ambiguous east asian character width configurable
  * treat ambiguous east asian character as fullwidth (= 2)
  * use inherits module to reduce browserify build size
  * update object-keys module
  * update estraverse and stringifier module