Add global toggle to use_cassette.

vcr/__init__.py

@@ -1,5 +1,6 @@
 import logging
 from .config import VCR
+from . import cassette
 
 # Set default logging handler to avoid "No handler found" warnings.
 try:  # Python 2.7+
@@ -9,6 +10,9 @@ except ImportError:
         def emit(self, record):
             pass
 
+def global_toggle(enabled=True):
+    cassette.use_cassette._enabled = enabled
+
 
 logging.getLogger(__name__).addHandler(NullHandler())