Implicitly add WORKON_HOME to `python.venvFolders`

[DonJayamanne]

From @zgoda on May 19, 2017 11:30

I work almost exclusively in virtualenvs and I'd like to be able to select interpreter from list of virtualenvs if both workon command and WORKON_HOME envvar exist.

workon is shell command installed by virtualenvwrapper. When invoked without argument it lists available virtualenvs under its control, in alphabetical order:

$ workon
bde
brewlog
cocos
...
tsp
twt
wsm
yg01

Now I have to set complete path to interpreter in form of $WORKON_HOME/<envname>/bin/python. An alternative is to get virtualenvs by listing subdirs of $WORKON_HOME (this is exactly what workon does).

Copied from original issue: DonJayamanne/pythonVSCode#968

[brettcannon]

This would also help with pipenv who reads from WORKON_HOME as an alternative location to place virtual environments.