Allow more complex filtering

[dwhenry]

This PR duplicates the existing code then converts it from a module-based into a Class-based approach. The main reason for this to add extra functionality on the addition check (i.e. I want to be able to programmatically decide when to include additional based on the type of parent object and the current path)

This type of change was just not possible with the previous code implementation. I have reused the existing methods and avoided changing the interface so this can be used as a drop-in replacement for the previous code. The advantage of this approach is that rebasing changes from the master branch is do as should pick up any changes which are not to the core diffing method automatically.

[espadrine]

Hi @dwhenry!

Would it be OK to apply the changes to lib/json-diff/diff.rb? Your code organization looks good, and it seems backwards-compatible; am I wrong?

In terms of versioning, it feels awkward to have both a v2 and a version 0.5.0-dwhenry.