more viz + build fixes (#703)

* viz docs

* writing vizualization docs

* documenting plot_util

* docstrings in plot_util

* autopep8 and flake8

* spelling (using default vim spellchecker and ingoring things like dataframe, docstring and etc)

* rephrased viz.md a little bit

* more examples of viz code usage in the docs

setup.py

@@ -11,6 +11,7 @@ extras = {
     'test': [
         'filelock',
         'pytest',
+        'pytest-forked',
         'atari-py'
     ],
     'bullet': [