Ignore missing variables in Dataset.drop

Ignore missing variables when concatenating datasets?
Several users (@raj-kesavan, @richardotis, now myself) have wondered about how to concatenate xray Datasets with different variables.

With the current xray.concat, you need to awkwardly create dummy variables filled with NaN in datasets that don't have them (or drop mismatched variables entirely). Neither of these are great options -- concat should have an option (the default?) to take care of this for the user.

This would also be more consistent with pd.concat, which takes a more relaxed approach to matching dataframes with different variables (it does an outer join).