This plugin allows users to subscribe to pages, and emails them when
those pages are created or changed.

It needs the [[!cpan Mail::Sendmail]] perl module, and sends mail
using the local MTA.

Each user can configure which pages they are interested in, using an
[[ikiwiki/PageSpec]] on their Preferences page. Any change to a page
matching the PageSpec will send an email that includes the new content of
the page, and a link to the page on the web.

To make it easy to subscribe to comment threads when posting a comment,
or a page, there is a check box that can be used to subscribe, without
needing to manually edit the [[ikiwiki/PageSpec]].
