Add a config flag run in add-on finding mode only

[tomreece]

This was suggested by Brad:

Is it possible to add a config flag to only search for addon trades? I recently ran into a situation where I didn't want to initiate new trade partners but would not have minded if pucauto was still looking for addon trades.

[eengstrom]

I second that. Would be willing to work on it as well. Pretty trivial, especially in the update (#66) I provided, but I'm not sure the status of that yet.