Add key definition for printer sequences

These new combinations generate the same behaviour (basically) of
vt102. It is a good way of communicating st with other programs.

[0] http://www.vt100.net/docs/vt102-ug/chapter2.html

config.def.h

@@ -107,6 +107,8 @@ static Mousekey mshortcuts[] = {
 
 static Shortcut shortcuts[] = {
 	/* mask                 keysym          function        argument */
+	{ ControlMask,          XK_Print,       toggleprinter,  {.i =  0} },
+	{ ShiftMask,            XK_Print,       printscreen,    {.i =  0} },
 	{ MODKEY|ShiftMask,     XK_Prior,       xzoom,          {.i = +1} },
 	{ MODKEY|ShiftMask,     XK_Next,        xzoom,          {.i = -1} },
 	{ ShiftMask,            XK_Insert,      selpaste,       {.i =  0} },