Implement dynamic Skip and Specflow ignore

[JoshKeegan]

See #102 and also relevant to the older question #74. Outstanding tasks:

• [x] Refactor skip check to be outside of blocking message bus implementation
• [x] Documentation
• [x] Version bump (release prep)

In addition to those, it might be worth looking into the dynamic skip implementation in xunit3. If they have come up with a clean contract on how to trigger a skip it might be worth emulating it here.

[JoshKeegan]

xunit3's implementation is here: https://github.com/xunit/assert.xunit/commit/e6a6d5d22bbc7097f8decad5b3c8cac8cf3fb386#diff-ea2f3de6d3b7f7a60e769c4e56295280af756e310a13bd1477337f96ba002015

I like their contract (Assert.Skip(string reason)) but we couldn't implement it like that because we don't have the ability to modify the partial class. Other than the class & method name, there's nothing for us to really be compatible with so what's already here is fine and can just be removed when xunit3 goes mainstream.