Allow plotting backend option context without matplotlib
Right now, using `pd.option_context` to set the backend fails on `__exit__` when matplotlib isn't installed.

```python

In [1]: import pandas as pd

In [2]: with pd.option_context('plotting.backend', 'test'):
   ...:     pass
   ...:
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
~/sandbox/pandas/pandas/core/config_init.py in register_plotting_backend_cb(key)
    593         try:
--> 594             import pandas.plotting._matplotlib  # noqa
    595         except ImportError:

~/sandbox/pandas/pandas/plotting/_matplotlib/__init__.py in <module>
      2
----> 3 from pandas.plotting._matplotlib.boxplot import (
      4     BoxPlot,

~/sandbox/pandas/pandas/plotting/_matplotlib/boxplot.py in <module>
      3
----> 4 from matplotlib.artist import setp
      5 import numpy as np

ModuleNotFoundError: No module named 'matplotlib'

During handling of the above exception, another exception occurred:

ImportError                               Traceback (most recent call last)
<ipython-input-2-08ebc0f8d5ad> in <module>
      1 with pd.option_context('plotting.backend', 'test'):
----> 2     pass
      3

~/sandbox/pandas/pandas/_config/config.py in __exit__(self, *args)
    410         if self.undo:
    411             for pat, val in self.undo:
--> 412                 _set_option(pat, val, silent=True)
    413
    414

~/sandbox/pandas/pandas/_config/config.py in _set_option(*args, **kwargs)
    134             if silent:
    135                 with warnings.catch_warnings(record=True):
--> 136                     o.cb(key)
    137             else:
    138                 o.cb(key)

~/sandbox/pandas/pandas/core/config_init.py in register_plotting_backend_cb(key)
    595         except ImportError:
    596             raise ImportError(
--> 597                 "matplotlib is required for plotting when the "
    598                 'default backend "matplotlib" is selected.'
    599             )

ImportError: matplotlib is required for plotting when the default backend "matplotlib" is selected.

```

I don't believe that should raise an error. We may need to make the default option `None`, and then resole that to matplotlib later on.