Small change to init_app() to allow passing the dns

This is for cases where the config isn't available until the app is constructed. Otherwise, you have to do:

```python
if app.config.get('SENTRY_DSN'):
        sentry.dsn = app.config.get('SENTRY_DSN')
        sentry.init_app(app)
```

Instead you should be able to do:

```python
sentry.init_app(app, dsn=app.config.get('SENTRY_DSN'))
```