Add IHtmlGenerator and TextAreaTagHelper

[dougbu]

• part of Tag Helpers: create a new project in the MVC repository to hold <textarea/> and other MVC tag helpers #1243 (kind-of) and most of Tag Helpers: create a new project in the MVC repository to hold <textarea/> and other MVC tag helpers #1243
• DefaultHtmlGenerator 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 Microsoft.AspNet.Mvc.TagHelpers project
• add TextAreaTagHelper

[dougbu]

/cc @NTaylorMullen

• "[Design]" PR primarily because HtmlHelper doesn't yet use an IHtmlGenerator
• missing anti-forgery support in IHtmlGenerator
• will likely file a Task to add missing XML docs (mostly copies from IHtmlHelper w/ slight edits)

[NTaylorMullen]

👍 should I just inject the AntiForgery class for now? Assuming you'll add the MvcServices DI registration piece in the non-design piece

[dougbu]

• I just pushed the new project outline and one tag helper
• in next update, will add DI registration and IAntiForgery -- that should be done consistently
• saving HtmlHelper changes 'til last since you don't depend on that
• what "non-design piece"? this is all runtime code

[NTaylorMullen]

Add a version

[dougbu]

👌

[NTaylorMullen]

Also, add this project to the Mvc.NoFun.sln 😄 VS exploding at home more than I like haha

[dougbu]

@natemcmaster updated. haven't added project.json version yet but have added anti-forgery.

/cc @harshgMSFT @GrabYourPitchforks please take a quick look at change to AntiForgery.GetHtml() signature. any issues w/ returning TagBuilder and not HtmlString?

[GrabYourPitchforks]

I have no concerns re: the signature change.

[dougbu]

• updated commits, PR description, and title
• rebased and checked into TagHelpersFeature branch
• leaving this open for additional issues and because <textarea/> tag helper needs unit tests

[dougbu]

Will open a new PR to complete this work later today...