Plugin binary wrappers

At this time, to use Conduit we need to define plugin configuration in Conduit.yml and plugins need to be in Conduit repo. It would be nice if Conduit could also run a plugin binary.

```yaml
processors:
- name: processor_binary_wrapper
config:
path: /path/to/binary
```

Define IO scheme for passing data to binary.
Different plugins for daemon and one time calls?

Something similar to this:
https://github.com/influxdata/telegraf/tree/master/plugins/processors/execd