Add value_counts() method to SpanArray to support DataFrame.describe()

[frreiss]

DataFrame.describe() on a DataFrame with a span column currently doesn't work because our array types are missing the method value_counts(). The specific stack trace looks like this:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-55-f041b2b916b3> in <module>
----> 1 syntax_df.describe()

[...]

~/opt/miniconda3/envs/pd/lib/python3.7/site-packages/pandas/core/algorithms.py in value_counts(values, sort, ascending, normalize, bins, dropna)
    736 
    737             # handle Categorical and sparse,
--> 738             result = Series(values)._values.value_counts(dropna=dropna)
    739             result.name = name
    740             counts = result._values

AttributeError: 'CharSpanArray' object has no attribute 'value_counts'

We should implement the value_counts() method, following the example of the implementation for Pandas' built-in IntervalArray type.

Note that TokenSpanArray is currently a subclass of CharSpanArray, but the implementation of #91 may change that relationship.