* src/filebuffer.{cc,h} (FileBuffer::find_matches): Add a feedback
slot parameter and invoke it for every matching line.  The current
line number and the subject string are passed as arguments to the
feedback slot.

* src/filetree.{cc,h} (FileTree::signal_feedback): New signal.
(FileTree::find_matches_at_path_iter): Pass a feedback slot to
FileBuffer::find_matches() that forwards to signal_feedback()
with the fileinfo object bound as third argument.

* src/mainwindow.{cc,h} (print_location): New handler for the
feedback signal which prints the location of the match to standard
output, in the same format as used by 'grep -n'.
(InitState::feedback): New bool member.
(MainWindow::initialize): Connect the filetree's signal_feedback
to print_location if the init state's feedback flag is true.

* src/main.cc (parse_command_line): Add the --line-number command
line argument, intended to work just like the grep equivalent.