See issue #569. It would be nice to be able to enter custom display formats in the Edit Display format dialog when the format you require isn't part of the list. Bonus points for remembering custom display formats and having an option to give them names etc.

There are two problems here though. Both requiring us to add some more error checking code:

1. If the user types in some invalid code we'd get a syntax error. Not sure how we'd react there but we definitely don't display a proper error message.
2. The user could type in some code that generates more than one column. This one is tricky because it doesn't produce a syntax error but would cause a lot of confusion throughout the application. So by some means we'd have to detect this case and prohibit it.