Support explicitly setting a dimension order with to_dataframe()

[shoyer]

As discussed in #2346, it would be nice to support explicitly setting the desired order of dimensions when calling Dataset.to_dataframe() or DataArray.to_dataframe().

There is nice precedent for this in the to_dask_dataframe method:
http://xarray.pydata.org/en/stable/generated/xarray.Dataset.to_dask_dataframe.html

I imagine we could copy the exact same API for `to_dataframe.