Add TagHelperSample.Web

Add Tag Helper sample, #1242
- at least initially, input tag helper will ignore the "format" bound attribute if "type" is checkbox, hidden, password, or radio.
- `type="radio"` (not in this sample) needs a value to compare to the expression and the "value" HTML attribute Just Works(tm). so under the covers, that may be the only case where MVC tag helpers use `TagHelperContext.AllAttributes`