Support full set of variables in cmd

[User087]

The set of available variables is currently somewhat limited. I suggest adding support for the full set of variables available in VSCode, and which are accessible in e.g. tasks.json (https://code.visualstudio.com/docs/editor/tasks#_variable-substitution). This would e.g. allow using the ${command:python.interpreterPath} variable to write a command to activate the virtual environment, allowing subsequently running any commands that require the virtual environment, thus potentially mitigating #48.

[kortina]

Seems like a good idea to me! Would love help with this one.