Test: Command line extension management

From #691
- [x] Windows - **@aeschli**
- [x] OS X - **@isidorn**
- [x] Linux - **@dbaeumer**

The Code command line alias now has the power to list, install and uninstall extensions. Syntax:

```
code --list-extensions
code --install-extension <extension>
code --uninstall-extension <extension>
```

Try to break it. Try using it with a different language, see if NLS works.
