If the external id is precised, it will be used to dynamically retrieves
the URL when printing this content. This is useful for some source that
change the content's URL (like soundcloud).

Fuck soundcloud.

For now only allowed sites.

And this is probably just the begining… but this is the goal of
RaisErrors.

Everyone in is namespace and the subroutines will be safe.

The date and the sender_irc are attached to the channel, not the
content.