Add directive to allow timeout before redirecting

Inserts an optional timeout before redirecting that link. This gives the
browser enough time to fire requests before redirecting.

Default timeout is 300ms but users can override this.

Based on: https://segment.io/docs/libraries/analytics.js/#track-link
Fixes: https://github.com/luisfarzati/angulartics/issues/148

Totally open to suggestions. I'm using this in production already as it's necessary for tracking our link clicks that have browser redirects (ones not using angular internal routing).