[tox] add 'coverage' toxenv to make local html coverage report

This is useful if one wants to run coverage tests locally, and get a nice HTML report while one is writing tests.

Just run:

$ tox -e coverage -- {optional pytest arguments}

And you get a "htmlcov/index.html" to view in your browser.

tox.ini

@@ -16,12 +16,21 @@ commands =
     # run the test suite against the package installed inside tox env
     py.test {posargs:--pyargs fontTools}
 
+[testenv:coverage]
+basepython = {env:TOXPYTHON:python3.5}
+deps =
+    {[testenv]deps}
+    pytest-cov
+skip_install = true
+commands=
+    # measure test coverage and create html report
+    py.test --cov --cov-report html {posargs}
+
 [testenv:coveralls]
 passenv = TRAVIS TRAVIS_JOB_ID TRAVIS_BRANCH
 basepython=python3.5
 deps =
-    {[testenv]deps}
-    pytest-cov
+    {[testenv:coverage]deps}
     coveralls
 skip_install = true
 ignore_outcome = true