Feature request: setting lower and upper bounds for sleep duration

Hello. Nice projects, thanks for your efforts!

I may suggest one useful feature to have: custom sleep duration between following, liking, unfollowing etc. Right now many actions have some sleep intervals which in majority are not controlled by user e.g. when you navigate to next post etc. Would be nice to have something like `set_follow_sleep(min=X, max=Y)` and `InstaPy` will `sleep(min + random()%(max-min))` seconds instead of predefined values. And same couple of methods for other frequently used actions where currently preset values are used.