add parameter ignored_exception_types to the tornado client to allow ignore specific exceptions

[flaneur2020]

We've deployed sentry to our website in tornado recently, but soon get flooded with exceptions like HTTPError: HTTP 404: Not Found. It would be cool for us to ignore them.

There's a option RAVEN_IGNORE_EXCEPTIONS in the flask client, but haven't the equivalent in the tornado client yet. This PR is a copycat about this option:

        app.sentry_client = AsyncSentryClient(
            '__DSN__',
            ignored_exception_types=[HTTPError],
        )

Regards.