Filters/Outputs can optionally exit without shutting down heka

This is the first pass, I'd like to get some review on it, and I've tested it
with a few different plugins, both filters and outputs, and it seems to work.

I'd appreciate it if someone else could put it through its paces to make sure
I haven't overlooked anything.
Yeah, I originally had my `defer foRunner.exit(pConfig)` line up a bit higher, before this. But your right, this case should specifically cause a shutdown without the canExit behavior.