feature request: easy way to deal with counterbalancing across conditions

just finished implementing support for nested `listgroup` kwarg, so that users can specify a unique listgroup for each subject. This will be useful when conditions are randomized/counterbalanced across lists over subjects.

It would be nice to have a shortcut for counterbalanced designs. I propose that we support listgroup dictionaries, something like:

```
listgroup = {
'cbs' : [['random']*4+['forward']*4],['forward']*4]+['random']*4]],
'subj_cb' : [0,1]
}
```
internally, this would generate a nested `listgroup`, but would be less code on the user end to set it up. thoughts?