Tag Helpers: Add design time projections for attributes

[NTaylorMullen]

Right now when a TagHelper specifies an attribute should be more than just an HTML string we don't project the users code into the correct location within the generated C# file.

AKA:

public class SomeTagHelper : TagHelper
{
   public int Foo { get; set; }
}

<some foo="5" /> <!-- The 5 is not mapped to the generated C# file and therefore appears as plain text. -->