Add HtmlAttributeNameAttribute for TagHelper properties.

- Added the ability to override the HTML attribute name for TagHelper properties.
- Tested functionality of overriding the attribute name target and inheriting the attribute name.

#121
this is about expectations so nothing is "real". but in any case, my follow-up suggestion was really a nit and I should have made that clear.
FYI I typed in that nit because as-is readers will think everything about these `PropertyInfo` instances matters. `Mock<PropertyInfo>` would clarify what's actually important.