v0.2.0

New Features

• Added add_new/2 which only adds an item if it is not already present
• Added add_new!/2 which adds an item if it is not already present, or raises if it is
• Added remove!/2 which removes an item if it exists, or raises if it doesn't

Bug Fixes

• Fix some incorrect references to __MODULE__ in docs
• Correctly update linkages and move value to then end when calling add/2
  with an existing value