=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:


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.


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.