Added StepExecutor class to allow users to add custom logic before/after each step.

See discussion in pull request #164 .
It sounds like that would be a good idea, but to be honest I'm not sure I understand the way the async void use-case works well enough to comment. If it helps, I've uploaded an initial sample of the Visual Studio integration Test assembly that uses the code in this pull request. [Here is a sample test](https://github.com/OmerRaviv/BDDfyVSIntegrationTestSample/blob/master/src/Integration%20Tests/StartDebuggingTest.cs) and the aforementioned [BDDfy configuration](https://github.com/OmerRaviv/BDDfyVSIntegrationTestSample/blob/master/src/BDDfyConfiguration.cs).  You'll need VS2013SDK if you want to run this locally.