Add support for accessing the instance in converters

[Luc1412]

Currently it's not possible to access the current instance in a converter, except you use decorators. I currently define converters automatically based on their type with the field_transformer hook, unfortunately this only provides the class.

This can be implemented in multiple ways.

• Enforcing the self argument in converters. This would cause a breaking change)
• Only passing the instance if the user provides 2 arguments
• Passing the instance in field_transformer

[Luc1412]

Seems like #404 solves this issue. Unfortunately it seems to have fallen into oblivion