Add support for platform-specific target dirs

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.

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

or

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

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