Option for combine_attrs with conflicting values silently dropped

[shoyer]

merge() currently supports four options for merging attrs:

    combine_attrs : {"drop", "identical", "no_conflicts", "override"}, \
                    default: "drop"
        String indicating how to combine attrs of the objects being merged:
        - "drop": empty attrs on returned Dataset.
        - "identical": all attrs must be the same on every object.
        - "no_conflicts": attrs from all objects are combined, any that have
          the same name must also have the same value.
        - "override": skip comparing and copy attrs from the first dataset to
          the result.

It would be nice to have an option to combine attrs from all objects like "no_conflicts", but that drops attributes with conflicting values rather than raising an error. We might call this combine_attrs="drop_conflicts" or combine_attrs="matching".

This is similar to how xarray currently handles conflicting values for DataArray.name and would be more suitable to consider for the default behavior of merge and other functions/methods that merge coordinates (e.g., apply_ufunc, concat, where, binary arithmetic).

cc @keewis