Option to show copyright status of the logs themselves

Svetlana A. Tkachenko requested this via email:

> Would you please write a piece of code to ask people who use your software to specify the licence they would like to release the irc logs under? Otherwise it is an implied 'all rights reserved' which is bad. I am sure many of them are copyleft resources (allowing redistribution and modification of the logs) but forget to specify that.

I think the best solution would be to have a proper configuration file where I could specify arbitrary footer text.

The issue names no files, tests, or configuration entry point. Start by finding how irclog2html accepts options and renders HTML footers, then clarify the configuration format and expected copyright display; done means the selected log licence or arbitrary footer text is reflected in generated HTML.

Written by the indexing model from the issue text.