Moodle : new plugins

Hi,

2 new plugins for Moodle.
The first one is a multigraph plugin which display informations about files data (number, size)
The other display number of connected and active users in chat sessions
/etc/munin/plugin-conf.d/munin-node is owned by the packaging, please do not encourage people to make changes to it.

/etc/munin/plugin-conf.d/moodle.conf would be a good path.