Allow config to set user-agent used for request

Using a highly unique user agent is recommended by Reddit's API documentation as a way to decrease the amount of throttling. This shouldn't be a problem as the documentation says that a user is limited to 30 requests per minute, but I'm still seeing 429 responses from Reddit when opening cortex. This should help a bit, and isn't an intrusive change as it falls back to the "old" behavior when no user-agent  is specified.