Support external exporters

[ncabatoff]

The first version I wrote used an external exporter (i.e. spawned subprocess). The executable load_exporter is a vestige of that. I dropped that to simplify things, as there are many fewer error cases with an internal exporter. But running an existing exporter is a valid use case and should be supported.

The idea is that config specifies a command line to spawn an external exporter, details TBD.