Returns a raven-clj Sentry appender.

Requires the DSN (e.g. "https://<key>:<secret>@sentry.io/<project>")
to be passed in, see Sentry documentation for details.

Timbre's `*context*` will be passed to Sentry as `:extra` data. When logging
an exception with ex-data attached, it will be stringified and added under
`:ex-data` key (unless that key already exists in context).

Common options:
  * :tags, :environment, :release, and :modules will be passed to Sentry
    as attributes, Ref. https://docs.sentry.io/clientdev/attributes/.
  * :event-fn can be used to modify the raw event before sending it
    to Sentry.