Ability to set relative tresholds.

**Describe the solution you'd like**
I would love to have the ability to be able to set a relative treshold for notifications. For exapmle, it could sent a notifications if the speeds you're getting are x% or less than what you should be getting from your isp. Or if your latest result was x% slower than the previous one.

Tanks for all the amazing work you have already done!