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.
This is a bug b/c there's no way to distinguish btn a false that was explicitly specified in the config and a false b/c the user didn't specify a value. You need to declare PluginGlobals.CanExit as a *bool instead of a bool so you can tell the difference btn a nil and an explicit false (see PluginGlobals.UseFraming).