Add support for Specflow

[theramis]

Is your feature request related to a problem? Please describe.
Snapper does not have native support for Specflow at the moment. Currently Snapper uses the underlying implementation of Specflow which can be xunit, nunit, etc
This limits the usage of Snapper in Specflow.

Describe the solution you'd like
Native support for Specflow. It should support normal specflow tests as well as Scenario Outline tests (which are essentially XUnit Theory tests).
This could be done via a new nuget package or through extending the main Snapper nuget package.