filter module: ability to configure variables in the X-line reason, like $nick

[Robby-]

Description
The ability to configure variables in X-line reasons, like $nick (which at the moment is the only one I'd really like and that we could use). $nick would be replaced by the nickname of the user that triggered the filter.

Why this would be useful
It would make administration a bit simpler when we need to look up a specific X-line set by a filter. Often we get requests (by email, etc) about bans set by a filter by users who do not always directly provide all relevant info, e.g. the exact reason they see when trying to reconnect or their IP at the time, etc, but they may provide their nickname still, and we can use that to look up the X-line (by using the xlinetools module) without necessarily having to ask follow-up questions first.

An example filter tag using $nick in the reason could look like this:

<keyword pattern="*bannedpattern*"
         reason="Unwanted spam ($nick)"
         action="gline"
         flags="*"
         duration="3d"
>

For this example we assume the user's nick is BadUser, this would result in an X-line reason like Unwanted spam (BadUser) shown to the user when they get banned and when they attempt to reconnect.