[feature] condition: number increase/decrease absolute/relative

**Version and OS**
Docker 0.51.4

**Describe the solution you'd like**
To detect number increase and decrease, absolute (+/- x) and relative (+/- %).

**Describe the use-case and give concrete real-world examples**
Certain scenarios require manual evaluation (thus webpage processor). Examples are websites listing second hand items and websites listing real estate. An increase in listings is the moment to inspect, a decrease means listings are removed, and so ideally you don't get a notification for the latter. (Of course, when a list ánd remove happen between checks, you could miss it, that is the chance to take.)

The issue mentions the webpage processor but does not identify files, tests, or an entry point. Start by locating the webpage processor and existing change-notification conditions, then define how absolute and relative increases or decreases should affect notifications and validate the behavior with focused tests.

Written by the indexing model from the issue text.