Add config option for word delimiters

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

config.def.h

@@ -15,3 +15,9 @@ static const char *outbgcolor  = "#00ffff";
 static const char *outfgcolor  = "#000000";
 /* -l option; if nonzero, dmenu uses vertical list with given number of lines */
 static unsigned int lines      = 0;
+
+/*
+ * Characters not considered part of a word while deleting words
+ * for example: " /?\"&[]"
+ */
+static const char worddelimiters[] = " ";