Custom fill_value for from_dataframe/from_series

[shoyer]

It would be to have the option to customize the fill value when creating an xarray objects from pandas, instead of requiring to always be NaN.

This would probably be especially useful when creating sparse arrays (#3206), for which it often makes sense to use a fill value of zero. If your data has integer values (e.g., it represents counts), you probably don't want to let it be cast to float first.