Integrated terminal should use Python virtualenv

[gabeduke]

Environment data

VS Code version: 1.20.0-insider
Python Extension version: 0.9.1
Shell: zsh
Python Version: 2.7 (system), 3.*(virtualenv)
OS and version: Linux Mint 18.2 Cinnamon

Actual behavior

When a virtualenv is selected, after starting VS-Code the integrated terminal does not use the virtual environment.

Expected behavior

VS-Code integrated terminal should use the path to virtualenv python not the system default

This is a workflow bottleneck since a user often wants to run arbitrary python commands in the integrated shell, or start a python interpreter without sourcing the python virtualenv.

A simple solution would be a setting with the source path to use and append it to the specified shell rc file

Steps to reproduce:

1. python.venvPath is set to virtualenvs directory
2. python.pythonPath is set to virtual environment path
3. python --version in integrated terminal shows system version

[brettcannon]

Closing as a duplicate of #383 as we're already looking at providing support for what you're after.