document PY_COLORS and TOX_SKIP_ENV inside --help

[gaborbernat]

Resolves #163.

[codecov]

Codecov Report

Merging #1009 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1009      +/-   ##
==========================================
- Coverage   70.39%   70.39%   -0.01%     
==========================================
  Files          14       14              
  Lines        3483     3486       +3     
  Branches      465      465              
==========================================
+ Hits         2452     2454       +2     
- Misses        970      971       +1     
  Partials       61       61

Impacted Files	Coverage Δ
src/tox/session.py	78.12% <100%> (-0.06%)	⬇️

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