{DataArray,Dataset}.rank() should support an optional list of dimensions

{DataArray,Dataset}.rank() requires a single dim. Why not support an optional list of dimensions (defaulting to all)?

In [1]: import numpy as np, xarray as xr                                                                                                                                                                                                           

In [2]: d = xr.DataArray(np.arange(12).reshape((4,3)), dims=('abc', 'xyz'))                                                                                                                                                                        

In [3]: d                                                                                                                                                                                                                                          
Out[3]: 
<xarray.DataArray (abc: 4, xyz: 3)>
array([[ 0,  1,  2],
       [ 3,  4,  5],
       [ 6,  7,  8],
       [ 9, 10, 11]])
Dimensions without coordinates: abc, xyz

In [4]: d.rank()                                                                                                                                                                                                                                   
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-585571c1eca8> in <module>
----> 1 d.rank()

TypeError: rank() missing 1 required positional argument: 'dim'

In [5]: d.rank(dim=('xyz', 'abc'))                                                                                                                                                                                                                 
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-006c73551ff8> in <module>
----> 1 d.rank(dim=('xyz', 'abc'))

~/.conda/envs/build/lib/python3.7/site-packages/xarray/core/dataarray.py in rank(self, dim, pct, keep_attrs)
   3054         """
   3055 
-> 3056         ds = self._to_temp_dataset().rank(dim, pct=pct, keep_attrs=keep_attrs)
   3057         return self._from_temp_dataset(ds)
   3058 

~/.conda/envs/build/lib/python3.7/site-packages/xarray/core/dataset.py in rank(self, dim, pct, keep_attrs)
   5295         """
   5296         if dim not in self.dims:
-> 5297             raise ValueError("Dataset does not contain the dimension: %s" % dim)
   5298 
   5299         variables = {}

TypeError: not all arguments converted during string formatting

In [6]: xr.show_versions()                                                                                                                                                                                                                         

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.6 | packaged by conda-forge | (default, Jan  7 2020, 22:33:48) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 3.10.0-693.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.3

xarray: 0.15.0
pandas: 1.0.1
numpy: 1.18.1
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: 0.8.0
h5py: 2.10.0
Nio: None
zarr: None
cftime: 1.0.4.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.3.2
dask: 2.11.0
distributed: 2.11.0
matplotlib: 3.1.3
cartopy: None
seaborn: 0.10.0
numbagg: installed
setuptools: 45.2.0.post20200209
pip: 20.0.2
conda: 4.8.2
pytest: None
IPython: 7.12.0
sphinx: None

Start by reading the DataArray.rank and Dataset.rank entry points described in the issue, then reproduce the provided NumPy/xarray example. Check how rank currently validates dim and add coverage for no dimension, a list of dimensions, and the existing single-dimension behavior; done means the requested calls work without the shown TypeError.