recursively combine items from catalog using xarray.auto_combine

Xarray has a function called [auto_combine](http://xarray.pydata.org/en/stable/generated/xarray.auto_combine.html) which takes several datasets and combines them into one, using a series of heuristics to figure out the best way to merge. This is used internally in `open_mfdataset`.

It would be quite cool if I could take an intake catalog containing many xarray datasets in a hierarchy and open any point of this hierarchy into a single xarray dataset.

Sorry for the vague issue, but I just wanted to jot this idea down before I forgot it. Could be very useful in multiple contexts (e.g. https://github.com/NCAR/intake-siphon/pull/2).