Add posargs to tox testenv

Author: pganssle

tox.ini

@@ -11,7 +11,8 @@ deps =
     pytest-cov >= 2.0.0
     coverage
 commands = pytest --cov-config="{toxinidir}/tox.ini" \
-                  --cov="{envsitepackagesdir}/grid_strategy"
+                  --cov="{envsitepackagesdir}/grid_strategy" \
+                  {posargs}
 
 [testenv:coverage]
 description = combine coverage data and create reports