=head2 keywords

This module checks the Subject header of a message and ensures that it
starts (modulo an initial Re:) with a valid keyword.  By default, the
following keyword formats are supported:

    KEYWORD:
    KEYWORD/KEYWORD:
    [KEYWORD]
    [KEYWORD/KEYWORD]
    [KEYWORD][KEYWORD]

If multiple keywords are given, they all have to be valid.  Keywords are
checked against a list given in a file and are case-insensitive.

This module takes one optional argument, a reference to a sub which, if
given a subject line, returns a list of all the keywords present.  This
can be used to override the default patterns listed above.

There is one required configuration file directive:

=over 4

=item keywords FILENAME

Specifies the file from which the list of valid keywords will be read.
The file should list all valid keywords, one per line.

=back

Two failure messages are possible:

=over 4

=item No keywords found

There were no keywords in the subject header of the message.

=item Invalid keyword '%s'

A keyword was found that wasn't in the list of valid keywords.

=back