Update Result so it can consider different "ok" exit values

[bitprophet]

Right now, the runners.Result class defines whether a Runner execution succeeded or failed, in its .ok property-method. At the moment it's a hardcoded == 0; it should become configurable as in Fabric 1, so users can set up for expected nonzero exits without having to try/except.

Should involve:

• configuration value in the run tree
• kwarg in run
• passed into Result during result creation
• honored by Result.ok