This adds sort of a rate-limiting and pinging method preferences. If the
amount of players the person wants is reached or exceeded, they will be
automatically added to the !ping list; this is the default behaviour.
Doing !pingpref will make the bot automatically ping them shortly after
- this is done by a separate thread which is started on the first join
and exits when game starts or last player quits. Doing so will not
remove them from the ping list.
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