Support disabling tag helper processing at the element level

[NTaylorMullen]

There should be the ability to disable a specific element instance (in a CSHTML file) from being processed as a Tag Helper. e.g. consider that the input tag is mapped to the InputTagHelper class but I want a specific input element instance to be client-side only:

@addtaghelper "InputTagHelper"
@addtaghelper "DivTagHelper"

<input />  @* This will be processed as a tag helper *@

<!input /> @* This will *not* be processed as a tag helper and the '!' will be removed from the output HTML *@

<!div>     @* This will *not* be processed as a tag helper and the '!' will be removed from the output HTML *@
    Hello there
</!div>

If a prefix is also declared for the page with the same value, an exception should be thrown at parse time to indicate that the combination is invalid, e.g.:

@addtaghelper "InputTagHelper", prefix: "!"

@* Explodes at parse time *@

[dougbu]

nit: an <input/> tag helper won't have an issue in this area because the bound "for" attribute is an obvious switch. MVC tag helpers that might match somewhat accidentally include <a/> and <form/>.
since the problem is fairly specific, maybe the fix should just be in those tag helpers e.g. an "ignore-me" bound attribute -- not a general mechanism in Razor?

[yishaigalatzer]

The design is to only support ! hardcoded

<!input type="text" />

Means do not generate a TagHelper

We then strip the ! out. This only applies to tag helpers. We do not need to remove the ! from tags that are not tag helpers.