Add Count stat

[mwaskom]

This is like Hist, but simpler and potentially more obvious:

(
    so.Plot(tips, "size", color="time")
    .add(so.Bar(), so.Count(), so.Dodge())
)

[codecov]

Codecov Report

Merging #3086 (c0e67ed) into master (a02b6bf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3086   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files          74       74           
  Lines       23768    23802   +34     
=======================================
+ Hits        23387    23421   +34     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/_stats/aggregation.py	97.50% <ø> (ø)
seaborn/_stats/counting.py	100.00% <100.00%> (ø)
seaborn/distributions.py	96.35% <100.00%> (ø)
seaborn/objects.py	100.00% <100.00%> (ø)
tests/_stats/test_counting.py	100.00% <100.00%> (ø)