Adds support for instance attribute and custom attributes

[Axeltherabbit]

trafficstars

As I mentioned here https://github.com/shivanshs9/drf-problems/issues/10 I wasn't sure if there was already a way to do this, if there was never mind I guess.

example:

class Http400BadRequest(APIException):
    status_code = status.HTTP_400_BAD_REQUEST
    instance = "test/custom/thingy"
    extensions = {"errors": "custom stuff", "more custom things": "asd"}

[Axeltherabbit]

Are the changes fine? Can this be merged?