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.
- Dealing with conflicting content bheaviors is not dealt with in this PR, that's tracked here: https://github.com/aspnet/Razor/issues/164

#122
nit: not sure the compiler is smart enough to skip null-checking the `ContentBehavior` property.  either way, this looks like two `null` checks rather than the one you need.  so a ternary expression is more straightforward.