disas: add optional note support to cap_disas

Include support for outputting a note at the top of a chunk of
disassembly to capstone as well.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Message-Id: <20200513175134.19619-10-alex.bennee@linaro.org>