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).
how about:
```scala
def useLocalMode() { mode = Local(false) }
def useStrictLocalMode() { mode = Local(true) }
```
I kind of hate boolean parameters now unless the method name is super clear.