Add insert for the primary clipboard to MOD + Shift + Ins.

Thanks Mantas Mikulėnas <grawity@gmail.com> for the patch!

config.def.h

@@ -70,6 +70,7 @@ static Shortcut shortcuts[] = {
 	{ MODKEY|ShiftMask,	XK_Prior,	xzoom,		{.i = +1} },
 	{ MODKEY|ShiftMask,	XK_Next,	xzoom,		{.i = -1} },
 	{ ShiftMask,		XK_Insert,	selpaste,	{.i =  0} },
+	{ MODKEY|ShiftMask,	XK_Insert,	clippaste,	{.i =  0} },
 	{ MODKEY,		XK_Num_Lock,	numlock,	{.i =  0} },
 };