libtextstyle: New function styled_ostream_flush_to_current_style.
* gnulib-local/lib/styled-ostream.oo.h (struct styled_ostream): Add method
flush_to_current_style.
* gnulib-local/lib/fd-styled-ostream.oo.c
(fd_styled_ostream::flush_to_current_style): New method.
* gnulib-local/lib/term-styled-ostream.oo.c
(term_styled_ostream::flush_to_current_style): New method.
* gnulib-local/lib/html-styled-ostream.oo.c
(html_styled_ostream::flush_to_current_style): New method.
* libtextstyle/lib/textstyle.h (styled_ostream_flush_to_current_style): New
declaration.
* libtextstyle/doc/libtextstyle.texi (The styled_ostream class): Document
styled_ostream_flush_to_current_style.
* libtextstyle/NEWS: Mention the change.
