Optional arena numbering

[Former user]

Make arena numbering optional. So that abc2 is optionally unrelated to arena abc.

One possible implementation is to simply check for the directory abc2 before continuing to compute the base name of abc. This has the small downside of abc21 still being related to abc.

Suggested by <<user hakaku>> here: http://forums.minegoboom.com/viewtopic.php?p=80285#80285