add_members: Support for invite & pre-generated passwords

Here is a patch for the add_members command-line utility that adds support for inviting users and specifying pre-generated passwords.

If you specify a "--invite-only" option, the users are invited instead of being subscribed right away.

You can also specify pre-generated passwords as the last "word" (of non-whitespace characters) in address list files. For example:

John Doe <john.doe@example.com> PaSsWoRd
jane.doe@example.org pAsSwOrD
user.with.autogen.password@example.net