v3.0.0-rc.0

Breaking changes

• Some tweaks to the tokenizer/attribute selector parsing mean that whitespace
  locations might be slightly different to the 2.x code.
• Better attribute selector parsing with more validation; postcss-selector-parser
  no longer uses regular expressions to parse attribute selectors.
• Added an async API (thanks to @jacobp100); the default process API is now
  async, and the sync API is now accessed through processSync instead.
• Tweaks handling of Less interpolation (thanks to @jwilsson).
• Removes support for Node 0.12.

Other changes

• Set the parent when inserting a node (thanks to @chriseppstein).
• Correctly adjust indices when using insertBefore/insertAfter (thanks to @tivac).
• Fixes handling of namespaces with qualified tag selectors.
• Now accepts a PostCSS rule node for better syntax errors.
• Now more memory efficient when tokenizing selectors.