Add Preference to Toggle SHOWPLAN and DEBUG in Preview

I am getting a StackOverflow exception when I preview a particular REST service that has lots of data. There is a lot of query text in the AS console followed by a StackOverflow exception. See: https://gist.github.com/tejones/6250290

We need to be able to turn off SHOWPLAN and/or DEBUG to avoid this (and avoid the need for increasing the perm size).