fold tox environments

[graingert]

eg https://github.com/abusalimov/pytest-travis-fold

spec here https://blog.travis-ci.com/2013-05-22-improving-build-visibility-log-folds/

[ryanhiebert]

Sounds like an interesting feature! I'm not sure exactly what I'd suggest for where the folds should be, but I'd be open to a pull request to look at.

[graingert]

@ryanhiebert the folds should be between environments, toxenvs