Generalize configuration for text reports

The generated mailing list report has a header which provides basic info on the page.

However, in some cases this information is not enough.
For example, some lists may not allow posting; it would be useful to be able to explain why.
In the case of Apache Commons, the lists are shared, so users need to add a prefix to the subject line.
There are no doubt other use cases.

It would be useful to be able to override the introduction.