Add new tool

List a specific group of sn domains(r1 to r20) to save the time spent on
`update_hosts.py`. Suppose you know a new group domain
`r3---sn-i3beln7k.googlevideo.com` and want to know domains of this
whole group, use command `./list-sn-spec.py i3beln7k > i3beln7l.txt`

tools/list-sn-spec.py

@@ -0,0 +1,12 @@
+#!/usr/bin/env python
+
+import sys
+
+def main():
+    sn = sys.argv[1]
+    for k in range(1, 21):
+    	print 'r%d' % k + '---' + 'sn-' + sn + '.googlevideo.com'
+
+if __name__ == '__main__':
+    main()
+