Allow Random Values for --sleep-interval

[ap4125]

The --sleep-interval option is great. I think that this is the main reason why we can run downloads from heavily monitored sites.

Is there a way to modify this option so that it can accept random values? For example, instead of setting it to --sleep-interval 300 to set it to sleep for 300 seconds or 5 minutes between each download, make it like so:

--sleep-interval 120-300

so that the sleep interval would be set between 120 and 300 seconds.

The rationale is to simulate more realistic browsing behavior.

[ghost]

Having a browse here to see what I could actually work on.
Could we approach it like this?

sleep_interval = random.randrange(int(self.params.get('sleep_interval').split('-')[0]), int(self.params.get('sleep_interval').split('-')[1]))

I guess thats not ideal because we are taking a string as input and instead we should take two ints (and upper and a lower)?

Maybe @yan12125 can provide feedback on my noob approach 👍

Edit: I suppose we could just randomize the number in sleep_interval, sending pull request.