examples: mutt crypt-hooks generator

* Added a script which demonstrates how the groups module works.
* Script generates Mutt/Neomutt crypt-hooks for every group entry in
  gpg.conf, including those entries for multiple keys (Mutt handles
  that differently).

lang/python/examples/howto/mutt-groups.py

@@ -0,0 +1,64 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+from __future__ import absolute_import, division, unicode_literals
+
+# Copyright (C) 2018 Ben McGinnes <ben@gnupg.org>
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation; either version 2 of the License, or (at your option) any later
+# version.
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU Lesser General Public License as published by the Free
+# Software Foundation; either version 2.1 of the License, or (at your option)
+# any later version.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License and the GNU
+# Lesser General Public Licensefor more details.
+#
+# You should have received a copy of the GNU General Public License and the GNU
+# Lesser General Public along with this program; if not, see
+# <http://www.gnu.org/licenses/>.
+
+import sys
+from groups import group_lists
+
+"""
+Uses the groups module to generate Mutt crypt-hooks from gpg.conf.
+
+"""
+
+if len(sys.argv) >= 2:
+    hook_file = sys.argv[1]
+else:
+    hook_file = input("Enter the filename to save the crypt-hooks in: ")
+
+with open(hook_file, "w") as f:
+    f.write("""# Change settings based upon message recipient
+#
+#	send-hook [!]<pattern> <command>
+#
+# <command> is executed when sending mail to an address matching <pattern>
+#
+# crypt-hook regexp key-id
+#     The crypt-hook command provides a method by which you can
+#     specify the ID of the public key to be used when encrypting
+#     messages to a certain recipient.  The meaning of "key ID" is to
+#     be taken broadly: This can be a different e-mail address, a
+#     numerical key ID, or even just an arbitrary search string.  You
+#     may use multiple crypt-hooks with the same regexp; multiple
+#     matching crypt-hooks result in the use of multiple key-ids for a
+#     recipient.
+""")
+
+for n in range(len(group_lists)):
+    rule = group_lists[n][0].replace(".", "\\\\.")
+    with open(hook_file, "a") as f:
+        f.write("\n")
+        f.write("# {0}\n".format(group_lists[n][0]))
+        for i in range(len(group_lists[n][1])):
+            f.write("crypt-hook {0} {1}\n".format(rule, group_lists[n][1][i]))