libtextstyle: New function html_ostream_flush_to_current_style.

libtextstyle: New function html_ostream_flush_to_current_style.

* gnulib-local/lib/html-ostream.oo.h (struct html_ostream): Add method
flush_to_current_style.
* gnulib-local/lib/html-ostream.oo.c (html_ostream::flush_to_current_style): New
method.
* libtextstyle/lib/textstyle.h (html_ostream_flush_to_current_style): New
declaration.
* libtextstyle/doc/libtextstyle.texi (The html_ostream class): Document
html_ostream_flush_to_current_style.
* libtextstyle/NEWS: Mention the change.