add support for a logger function in errprinter (#681)

* add support for a logger function in errprinter

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

* moved print to stderr to logger function

* monkey patching logger will replace the print to stderr functionality instead of supplement it

Author: wsilva32

dronekit/util.py

@@ -3,5 +3,8 @@
 
 
 def errprinter(*args):
+    logger(*args)
+
+def logger(*args):
     print(*args, file=sys.stderr)
     sys.stderr.flush()