Add support for platform-specific target dirs

[fwcd]

Currently, when createTargetDirIfNeeded is true and the directory does not exist, the first targetDir will be created. Usually, when there are several, there is a 'correct' target directory to create (e.g. ~/.config/... vs ~/AppData/...). Thus it would be good to scope these by platform, e.g.

{
  "targetDir": {
    "linux": [
      ...
    ],
    "darwin": [
      ...
    ],
    "windows": [
      ...
    ]
  }
}

or

{
  "targetDir": {
    {
       "path": "...",
       "platforms": [
         ...
       ]
    },
    ...
  }
}

In any case, we should retain compatibility with the current targetDir: string[] syntax.