Add support for Categories/Traits

[biltongza]

The built in test runner in Visual Studio allows you to group tests by trait. Different frameworks implement this with different attributes, for example NUnit uses the [Category("Category Goes Here")] attribute. We use this to separate out our unit tests and integration tests, so we can easily deselect integration tests when we want to do a quick sanity check after a code change (since the integration tests run for longer).

It would be great if we could do the same thing with AxoCover, since AxoCover seems to be the only code coverage tool that works in our environment.

[m-ringler]

One relatively simple option might be to make the targetargs: of OpenCover configurable via the Settings GUI (see https://stackoverflow.com/questions/31818921/opencover-how-to-exclude-tests-with-certain-category)