Add support for custom serializers

[jdkizer9]

Some of my models that I would like to audit contain sensitive information that'd prefer not be saved. Ideally, I would have the option of fields to ignore on a per model basis when performing json serialization. I think that this could be accomplished with something as simple as a callback that post serialization that would allow for modifying the json. Additionally, for changes to sensitive fields, it would be nice to see that a change to that field was made without showing the info itself.

[jdkizer9]

Opened #93 to address this. Closing