Add option to handle implicit keyword in implicit parameter lists

[jkinkead]

Today, parameter lists marked implicit have the keyword forced to its own line:

def foo(
  a: Int,
  b: Int
)(
  implicit // On its own line.
  ec: ExecutionContext,
  system: ActorSystem
): Unit

There should be an option to Preserve / Prevent / Force this behavior.

This was introduced in the daniel-trinh fork, and should be fixed in 1.0.

Per comment on #206 from @jrudolph .