tox.ini: add {posargs} to pass additional command line arguments to py.test

When invoking the tox command, the arguments after the -- will be passed on to the py.test command:

For example:
$ tox -- -x -s -k 'test_something or test_other'

http://tox.readthedocs.io/en/1.0/example/general.html#interactively-passing-positional-arguments

tox.ini

@@ -17,7 +17,7 @@ commands =
     {envpython} -c "import sys; print(sys.version)"
     {envpython} -c "import struct; print(struct.calcsize('P') * 8)"
     # run the test suite
-    py.test
+    py.test {posargs}
 
 [testenv:coveralls]
 passenv = TRAVIS TRAVIS_JOB_ID TRAVIS_BRANCH