Support Theories / parameterizable benchmarks

[Aaronontheweb]

Allow benchmarks to be tested over a range of values, rather than just a static set.

[mcquiggd]

Just a heads up, but I do have a project that has supported Theories / parameters with NBench, via xUnit, for over a year now ;)

https://github.com/Pro-Coded/Pro.NBench.xUnit

Just had a pull request to update the referenced packages, which I should be publishing on Nuget tomorrow.

I am happy to continue working on this project to add features you may have identified. But, can understand if you prefer this to be part of the framework...

EDIT: By the way, there is an issue running NBench xUnit tests with Visual Studio Test Explorer, NBench Issue 161 which is also causing an issue in my project.