Carry coordinates

I noticed that all coordinates are lost when passing through xhistogram. 

Do you think [pydata/xarray#3453](https://github.com/pydata/xarray/pull/3453), would make it feasible to carry those (see [xgcm/xgcm#150](https://github.com/xgcm/xgcm/issues/150) for discussion), or is there a specific reason why the coordinates are dropped by xhistogram?