v2.2.0: minor api name change

Renames the UNDEFINED consts and func to be slightly more self-descriptive as to their intent.

v2.1.0: Callbacks

Added ability to provide a callback to handle the merging of values

v2.0.5: Improvements

• Improved object field merging
• Now internally tracking merge depth and context for use in error messages
• Internal API improvements

v2.0.4: Bugfixes & Features

• Added recursive array value comparison
• Fixed array_unique with non-scalar values
• No longer considering null to be "undefined" on object fields
• Internal cleanup

Initial Stable Release

This release is the first with guaranteed future API compatibility, providing basic top-level and recursive object merging.