Support for downloading for a platform other than the host platform.

This resolves #4 by allowing you to specify an alternate platform to download. For backwards compatibility it still places the downloaded shell in the main outputDir by default, but also adds an option to have it append the platform name to the path (e.g., binaries/darwin/) automatically.
I think we can remove the `addPlatformToOutputPath` option and assume that it is false. It would still work with Atom because it sets `atomShellDownloadDir = path.join(os.tmpdir(), 'atom-cached-atom-shells')`.