add support for a logger function in errprinter

* this enables other software to monkey patch the logger function to capture dronekit output and exceptions

dronekit/util.py

@@ -5,3 +5,7 @@
 def errprinter(*args):
     print(*args, file=sys.stderr)
     sys.stderr.flush()
+    logger(*args)
+
+def logger(*args):
+    pass