Marta here, from Trello Support.
Right now, that feature isn’t available within Trello. I’ll pass your interest in this on to our team for consideration—please let me know if there's any additional info you'd like for me to send their way.
If you're curious about how our team decides on what requests we work on, this article gives some insight into that process: http://help.trello.com/article/724-submitting-feature-requests-for-trello
Also, to see what features our devs are currently working on, you can check out our dev board at http://www.trello.com/dev.
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