Support absolute file:// paths in sourceMappingURL

Hi! I'm working on https://github.com/mozilla/tofino and we have some interesting requirements. It appears that we need absolute `file://` paths in our `sourceMappingURL` to support source maps in the (Chrome) devtools that Electron provides natively. That is, we want:

```
//# sourceMappingURL=file:///Users/nalexander/Mozilla/tofino/lib/main/browser.map
```

Unfortunately, that conflicts with `source-map-support`, which doesn't appreciate absolute paths of this kind. Would you merge a patch to handle (in Node.js only) `file://` protocols?