Keep custom columns in files

[Toppari]

Hi,

Would be it possible to stop gtfstidy from removing some custom column (not referenced in gtfs spec) e.g in stops.txt? Like if I would have column named "custom_id" in stops.txt, when running gtfstidy with some option it would not remove it.

[derhuerst]

Coincidentally, this would also solve #12 for me.

[hbruch]

+1 for a flag to keep custom columns, like e.g. feed_info.feed_id