Add ignore option to unstack or something like that
per convo with @jseabold

```
Well, it's only the ones that are the same that I want to ignore. I'd
say that's up to the user to do this right. No checking, just take the
first value or something, but that's up to you.

df = df.unstack('agefrom', ignore='ccode')

df = df.unstack('agefrom', keep='ccode')

df = df.unstack('agefrom', constant='ccode')

?
```

primary issue i can think will be what to do w/ hierarchical columns with an ignore argument-- should things get munged automatically? should the key for the ignored column be like `'ccode', ''` or `'ccode', None`?
