React to options and hosting changes
Add TagHelperSample.Web
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, ...
Add `IHtmlGenerator` to DI and enable MVC tag helpers in TagHelperSample.Web
- also fix a couple of minor issues in TagHelperSample.Web
Add `TextAreaTagHelper` and Microsoft.AspNet.Mvc.TagHelpers project
- rest of #1243 and all of #1244
- needs a few utility methods for copying `TagBuilder` to `TagHelperOutput`
 - but works as-is
Add tests of `TextAreaTagHelper`
- new Microsoft.AspNet.Mvc.TagHelpers.Test project
- bit of test infrastructure -- `TestableHtmlGenerator`
 - short-curcuits validation attribute and AntiForgery additions
 - should help when testing all MVC tag helpers
Update `HtmlHelper` to use `IHtmlGenerator`
- make a few more methods available as `internal static` in `DefaultHtmlGenerator`
 - remove `IHtmlGenerator.GenerateOption()`; now `internal static`

nits:
- add `IHtmlGenerator.IdAttributeDotReplacement`
- move `DefaultHtmlGenerator.IdAttributeDotReplacement` after constructor
- move `HtmlHelper.ActionLink()` below static methods
- move newly-`internal` methods together in `DefaultHtmlGenerator`
- correct placement of `DefaultHtmlGenerator.GetValidationAttributes()` comment
Reacted to options and hosting changes after rebase.
Add extensions for TagHelperOutput to copy from TagBuilder.

- Added tests to validate functionality of all TagHelperOutputExtension methods.

#1319
Addressed code review comments.