Check word prefixes to reduce problem space

Produce a prefix wordlist from a dictionary wordlist using the
`filter_prefix.sh` script.

filter_prefix.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+if [ -z $1 ] || [ -z $2 ]; then
+    echo "Usage: ./filter_prefix.sh WORDLIST PREFIX_LENGTH"
+    exit 0
+else
+    cat "$1" | pcregrep -o1 "^([a-zA-Z]{$2})" | uniq
+    exit 0
+fi