Implicit execution context / easier switching between modes

Would something like this makes sense? Is there a way to recover the hadoop config after parsing the args/mode, or would we need to preserve it at startup (like in this pr)?

Sriram pointed me to https://github.com/twitter/scalding/issues/989 which is related but more general (and more effort).
oh, i meant to add a default to this param but forgot. 

i like two methods better, switched to your suggestion.