Add config option for word delimiters

Let the user configure word boundaries other than ' ', only works with
the portable character set.

dmenu.c

@@ -314,9 +314,11 @@ keypress(XKeyEvent *ev)
 			insert(NULL, 0 - cursor);
 			break;
 		case XK_w: /* delete word */
-			while (cursor > 0 && text[nextrune(-1)] == ' ')
+			while (cursor > 0 && strchr(worddelimiters,
+			       text[nextrune(-1)]))
 				insert(NULL, nextrune(-1) - cursor);
-			while (cursor > 0 && text[nextrune(-1)] != ' ')
+			while (cursor > 0 && !strchr(worddelimiters,
+			       text[nextrune(-1)]))
 				insert(NULL, nextrune(-1) - cursor);
 			break;
 		case XK_y: /* paste selection */