Add Support For Arbitrary Message Parameters

[StrangeWill]

Rollbar's API Spec says we can have additional things on the message body, lets support adding those.

[StrangeWill]

Ideally I'd like to use a structure like in https://github.com/RoushTech/SegmentDotNet, but I'd really like to make that reusable and put it in a shared library or something, otherwise like half of my REST APIs are going to have this code copy/pasted into them...