Allow pipeline stage to connect another pipeline

I don't see this in my Github searches, so i figured I'd ask here. I ran into an issue with slookup (https://github.com/billmurrin/graylog-plugin-slookup-function/issues/9) for a specific pipeline rule that consistently failed to work (and dropped messages out of that stream to boot).

I had to work around this by separating these two when clauses into different stages, and place them into a completely separate pipeline. Due to other constraints, I had to also create a different stream to attach this pipeline to to ensure that the parsing was done before these alert condition rules.

It would be more flexible for complex conditions like these to be able to attach another pipeline to start at a specific stage of a processing pipeline rather than having to tag it into another stream to accomplish the same. Or perhaps a route_to_pipeline function that additionally begins processing the message through another pipeline (while finishing the original pipeline it's currently processing through, or even better a flag whether to continue on the original while sending it down the new pipeline or cut the original pipeline short).