Pass positional arguments to Tox environments commands.

tox.ini

@@ -23,12 +23,12 @@ commands =
 [testenv:black]
 basepython = python3.7
 commands =
-    black .
+    black {posargs} .
 
 [testenv:end-to-end]
 deps = -rtest_requirements.txt
 commands =
-    pytest tests/end-to-end
+    pytest {posargs} tests/end-to-end
 
 [testenv:isort]
 deps = {[testenv]deps}