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

[Robbin Ehn]

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