Hooks for XArray operations

In hope of cleaner dask and sparse support (pydata/sparse#1), I wanted to suggest hooks for XArray operations.

Something like the following:

try:
    import dask.array as da
    xarray.hooks.register('nansum', da.array, da.nansum)
    ...
except ImportError:
    pass


try:
    import sparse.SparseArray
    xarray.hooks.register('nansum', sparse.SparseArray, sparse.nansum)
    ...
except ImportError:
    pass

Functions would work something like the following:
(the register would fall back to Numpy if nothing is found)

• Check type of first/primary argument.
• Check register for function.
• Call function

I would argue that this should be in Numpy, but it's a huge project to put it there.

The issue names no files, tests, or entry points. Start by reviewing how XArray currently dispatches operations, then determine where a type-based hook registry and NumPy fallback would fit; done would mean optional dask and sparse implementations can register operations and receive the appropriate primary argument type.