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).
`useHdfsMode`?
```scala
println("To use HDFS/Hadoop mode, you must *start* the repl in hadoop mode to get the hadoop configuration from the hadoop command.")
```