Enable configuration override of job plugin defaults (tests not updated)

I spent ages trying to figure out:
1. How to configure strider-heroku's client id and secret
2. Why PLUGIN_HEROKU_CLIENT_ID and PLUGIN_HEROKU_CLIENT_SECRET didn't get through to strider-heroku

Finally realised lib/provider.js is the only one that overrides a plugin's appConfig with config.plugins.<plugin>.*.  So I've copied it's defExtend function and execution to lib/job.js.  I'm not familiar, yet, with Strider's plugin structure, so not sure if it needs to be spread to more places.

I also haven't written a test yet - maybe a kind soul could help me figure out how to test this particularly piece of functionality?

With this patch, you can now configure your heroku client id and secret with:
PLUGIN_HEROKU_CLIENT_ID and PLUGIN_HEROKU_CLIENT_SECRET