Make the TagHelperContext available to content of Tag Helpers

[DamianEdwards]

As part of #221, contents of all Tag Helper invocations will now be wrapped in a delegate to allow for conditional execution, retrieval, or even suppresion, of the content by the surround Tag Helper.

This delegate should accept a TagHelperContext tagHelperContext parameter that is passed in by the page and shared with the surrounding TagHelper. When combined with #238, this enables the content to either receive or pass values back to the surrounding Tag Helper (depending on the order of execution).

This feature would be used by the CacheTagHelper (aspnet/Mvc#1552) to enable the contents to add cache invalidation triggers.

e.g.

Tag Helper Declaration

[TagName("my-tag")]
public class MyTagHelper : TagHelper {
    public override void Process(TagHelperContext context, TagHelperOutput output) {
        context.Items["message"] = "This came from the MyTagHelper class!";
    }
}

Consumption

<my-tag>
    Message from the tag helper: @tagHelperContext.Items["message"]
</my-tag>