text: Title: Check for existing asyncio event loops after forking
  Body: It seems there are some cases where there is already an existing, active event loop after forking, which causes an exception when the child process tries to create a new event loop.  It should check for an existing event loop first, and only create one if there's no active loop available.
  
  Target function: https://github.com/jreese/aiomultiprocess/blob/master/aiomultiprocess/core.py#L93
  
  See #4 for context.