Add multi-root support

[SteveL-MSFT]

Enable EditorServices to work with microsoft/vscode#396

[TylerLeonhardt]

Thanks Steve! This is a good one for sure. I think this prompts a question of... what does support for this look like?

I think most of this support revolves around the integrated console. When you switch to a file in another root, should the console switch to that directory? Or perhaps there should be a PowerShell session - per root?

@daviwil @SeeminglyScience @rkeithhill - any thoughts?

[daviwil]

I can take this on, I know what's involved. It's also a good opportunity to implement multi-root workspace support for Atom's atom-languageclient :)

[TylerLeonhardt]

You can take the @daviwil out of the PSES... but you can't take the PSES out of the @daviwil. 😄