enable xarray_kwargs

[aaronspring]

likely connected to #26

currently there is no way to specify kwargs to be passed to xarray.open_dataset.

import intake_xarray

intake_xarray.NetCDFSource('simplecache::https://www.ncei.noaa.gov/thredds/fileServer/model-gefs-003/202008/20200831/gensanl-b_3_20200831_1800_000_20.grb2',
                           xarray_kwargs=dict(engine='cfgrib', backend_kwargs=dict(filter_by_keys={'typeOfLevel': 'meanSea'})),
).to_dask()

but we do not allow such xarray_kwargs in intake-thredds

[aaronspring]

How to fix: intake/intake-xarray#52