support multiple limits

[willkg]

The docs suggest to use the keys argument to support multiple limits:

@ratelimit(keys=lambda x: 'min', rate='1/m')
@ratelimit(keys=lambda x: 'hour', rate='10/h')
@ratelimit(keys=lambda x: 'day', rate='50/d')
def post(request):
    # Stack them.
    # Note: once a decorator limits the request, the ones after
    # won't count the request for limiting.
    return HttpResponse()

Without that keys argument, then all three limits use the same cache key and therefore whichever gets cleared first clears them all.

It'd be nice if stacking ratelimits worked better with cache keys.

[jsocol]

Closing in favor of #48.