The prototype for a custom printing callback.
level indicates what kind of information is being printed. Eg, it will be
data is header data.
direction is either '<', '>', or ' ', and
data is the single line to print; the printer is expected to add a
To get the effect of the default printer, you would do:
printf ("%c %s\n", direction, data);
the level of the information being printed.
a single-character prefix to
data to print
the data passed to set_printer