Reduction over multiple specified dimensions

We currently support
```Py
da.min()
da.min('x')
```

Could it be useful to also support passing a list of dimensions we want to reduce over, like the `da.bins.concat()` accepts a list of dims.
If you have 3d data, you could do `da.min(('x', 'y'))` and preserve the `z` dimension?

This is currently achievable via `da.min('x').min('y')` but I see no harm in supporting both?