Change scipy/statsmodels install extra to be [stats] and do some housekeeping

[mwaskom]

• Install optional statistical dependencies with pip install seaborn[stats]
• Consolidate lint / testing config in setup.cfg
• Change the development version string to match the beta releases
• Add a filetype extension for the LICENSE file
• Use an external URL for the logo in the README

[codecov]

Codecov Report

Merging #2939 (0fc5b98) into master (a259ac5) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 0fc5b98 differs from pull request most recent head c3b6d81. Consider uploading reports for the commit c3b6d81 to get more accurate results

@@           Coverage Diff           @@
##           master    #2939   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          69       69           
  Lines       22985    22985           
=======================================
  Hits        22596    22596           
  Misses        389      389           

Impacted Files	Coverage Δ
seaborn/__init__.py	100.00% <100.00%> (ø)