Add ContentBehaviorAttribute for TagHelpers.

- Added detection of custom ContentBehaviors via the ContentBehaviorAttribute in the TagHelperDescriptorFactory.
- Updated some comments in the ContentBehavior enum.
- Add tests to validate custom content behavior resolution.

#122
This also needs an aspnet50 build. net45 != aspnet50 and Microsoft.Framework.Runtime.Interfaces doesn't work on net45