Add options for `network.enabled` and `network.error`

Add options to match the TrackJS Browser agent by installing with:
```
TrackJS.install({
   token: '',
   network: { enabled: true, error: true }
});
```

`enabled` toggles whether or not we should record network requests in telemetry automatically.
`error` toggles whether an error should be recorded if the network request returns with a >=400 status code.

