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

[Kim Barrett]

> On Mar 9, 2016, at 3:35 AM, Robbin Ehn <robbin.ehn at oracle.com> 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.
> 
> Thanks!
> 
> /Robbin

Looks good.