Allow passing in custom logger

When used with a notebook, lets us pass in notebook's
configured logger.

simpervisor/process.py

@@ -16,14 +16,18 @@ class KilledProcessError(Exception):
     pass
 
 class SupervisedProcess:
-    def __init__(self, name, *args, always_restart=False, ready_func=None, ready_timeout=5, **kwargs):
+    def __init__(self, name, *args, always_restart=False, ready_func=None, ready_timeout=5, log=None,**kwargs):
         self.always_restart = always_restart
         self.name = name
         self._proc_args = args
         self._proc_kwargs = kwargs
         self.ready_func = ready_func
         self.ready_timeout = ready_timeout
         self.proc: asyncio.Process = None
+        if log is None:
+            self.log = logging.getLogger('simpervisor')
+        else:
+            self.log = log
 
         # asyncio.Process has no 'poll', so we keep that state internally
         self.running = False
@@ -39,8 +43,6 @@ def __init__(self, name, *args, always_restart=False, ready_func=None, ready_tim
         # signals is synchronous.
         self._proc_lock = asyncio.Lock()
 
-        self.log = logging.getLogger('simpervisor')
-
     def _debug_log(self, action, message, extras=None):
         """
         Log debug message with some added meta information.