Windows: replace all backslashes

In praise of the beloved languag, let's fix this.
```javascript
"foo\\bar\\baz".replace("\\","/")  // -> foo/bar\baz
"foo\\bar\\baz".replace(/\\/g,"/") // -> foo/bar/baz

```
Not sure whether this one is right here - `sourceMap.file` is already modified in the `write` function.