Update deep merge interface (api)

[RebeccaStevens]

Based off #215

Changes

• Cloning is now off by default
• A custom clone function can now be provided
  • This is primarily to handle the case of cloning non-plain objects
• Updated the exposed internals
  • add deepClone(value)
  • deep clones a value in the same way that cloneUnlessOtherwiseSpecified did except an options object isn't supplied, the same original one is used.
  • add deepMerge(x, y)
    • deep merges two values in the same way that top level deepmerge does except an options object isn't supplied, the same original one is used.
  • remove cloneUnlessOtherwiseSpecified(value)
  • function no longer exists - replaced by getSubtree (not exposed) and deepClone (exposed)

closes #212 fixes #201 closes #163