libtextstyle: html-ostream: Add hyperlink support.

libtextstyle: html-ostream: Add hyperlink support.

* libtextstyle/gnulib-local/lib/html-ostream.oo.h (struct html_ostream): Add
methods get_hyperlink_ref, set_hyperlink_ref.
* libtextstyle/gnulib-local/lib/html-ostream.oo.c (struct html_ostream): Add
field hyperlink_ref;
(write_attribute_value): New function.
(html_ostream::free): Emit an anchor end if needed.
(html_ostream::get_hyperlink_ref, html_ostream::set_hyperlink_ref): New
functions.
(html_ostream_create): Initialize the hyperlink_ref field.
* libtextstyle/lib/textstyle.h (html_ostream_get_hyperlink_ref,
html_ostream_set_hyperlink_ref): New declarations.