Add setting in product.json for customDataDirectory

[DickvdBrink]

This setting allows the user to provide a custom directory for extensions, snippets and settings.
This is the first step for making vscode a portable application.

refs #329

Feedback appreciated, feel free to close though :)

Note: Not there is still some stuff in the %appdata% directory; the LocalStorage, GPUCache, DevTools Extensions and a electron settings file

[msftclas]

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[bpasero]

@DickvdBrink thanks, this is however not enough. You will need to set the data directory very early, see https://github.com/Microsoft/vscode/blob/master/src/vs/workbench/electron-main/bootstrap.js#L24 for an example how we set it for development.

I think we should check what Atom is doing here for being portable (atom/atom#2939) and do something similar. At the very minimum, there should be a command line argument to set the data location and extensions location. Currently they are split but at some point we want to merge them into one. I would also think a good fallback is to put the directories next to code.exe if you run with something like "--portable".