RFR(xs): 8151265: Add a way to extend UL tags

[David Holmes]

Hi Robbin,

On 9/03/2016 6:35 PM, Robbin Ehn wrote:
> Hi, please review this small change.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8151265/
> Webrev: http://cr.openjdk.java.net/~rehn/8151265/webrev/
>
> This adds an extension hpp with macro for extension tags.

Looks okay to me.

Thanks,
David

> Thanks!
>
> /Robbin