New env command and sub commands (#731)

* Add a env:info command

* Add env:use command

* Fix tests on Windows

* Fix tests

* Fix tests

* Fix tests for Python 3.5

* Use a hash in virtualenv names dependent on the project’s path

* Add tests for the `env use` command

* Fix tests

* Add a `env list` command

* Fix venvs being recreated when switching minor Python versions

* Add a `env remove` command

* Update CHANGELOG

* Update documentation