Desired behavior:

When given a command line argument (perhaps -] or -m?) or a nanorc
option (perhaps "brackets" or "matchbrackets"), do "live" bracket
matching. That is, immediately when the user types a close bracket
(maybe an open brackets, too?), we try to find its matching open
bracket. If it is found and on screen, highlight the bracket (ideally,
briefly, meaning one second or when the next character is typed,
whichever is faster). If it is found off screen, output a status bar
message with the line that matched. If it is not found, output the "No
matching bracket" message.

Implementation:

Add winio.c: void highlight_char(filestruct *fileptr, int y, int x)
which highlights the character at x, y if it is on screen.
wechochar(WINDOW *win, chtype ch) is what to use instead
of waddch/wrefresh.

Add search.c: void do_flash_bracket(void) which duplicates a bunch of
the code from do_find_bracket() to find the matching bracket, if any,
but instead of going there, just flashes it (or displays the match or
lack thereof in the status bar). Details: displayed match should always
include the matching paren (we should try to truncate the displayed line
appropriately if this wouldn't happen).

Add global flag to enable this behavior, and recognize the command line
argument or nanorc option to turn the flag on.