Add `InputTagHelper`

- first part of #1245 (mostly needs tests)
add the ignorecase comparer.  There's some odd scenarios where if a user does `<input Type="Foo" />` it will be copied back onto the `TagHelperOutput` object with attribute name `Type` which in turn makes this evaluate to false. Granted this will all change after beta1 when https://github.com/aspnet/Razor/issues/186 is completed.