Add IHtmlGenerator and its default implementation

- part of #1243 (kind-of)
- mostly copied from `HtmlHelper` but refactored to
 - consistently take a `ViewContext` parameter and return a `TagBuilder`
 - provide `GenerateActionLink()` and `GenerateRouteLink()`
 - provide a separate `GenerateHiddenForCheckBox()`, allowing
   `GenerateCheckBox()` to return a `TagBuilder`
 - `GenerateForm()`'s `method` parameter is a `string`, not `FormMethod`

nits: format document, consistent line wrapping, variable name changes, ...

Author: dougbu

src/Microsoft.AspNet.Mvc.Core/AntiForgery/AntiForgery.cs

@@ -42,10 +42,10 @@ public AntiForgery([NotNull] IClaimUidExtractor claimUidExtractor,
         /// This method has a side effect:
         /// A response cookie is set if there is no valid cookie associated with the request.
         /// </remarks>
-        public HtmlString GetHtml([NotNull] HttpContext context)
+        public TagBuilder GetHtml([NotNull] HttpContext context)
         {
             var builder = _worker.GetFormInputElement(context);
-            return builder.ToHtmlString(TagRenderMode.SelfClosing);
+            return builder;
         }
 
         /// <summary>