Customize report layout

[soulseekah]

Provide filters to customize how the reports look and feel. This would be the beginning, afterwards more advanced things could be attempted perhaps.

[soulseekah]

A new filter called gf_digest_email_subject is now available. It is called with the original subject as the first filter argument, the form names (as an array) as the second one, an array( $from, $to ) denoting the date range of the leads, and finally the CSV filename on disk if CSV-mode is enabled, otherwise a null is passed.

See commit 73f5ff4