[feature] Limit "Only trigger when new lines appear" to examine last X snapshots

**Version and OS**
v0.39.17.2 docker

**Is your feature request related to a problem? Please describe.**
No

**Describe the solution you'd like**
"Only trigger when new lines appear" works great, but as described it "compares new lines against all history". Sometimes it can cause some problems when after a long time there is a new line which we want to fish out, but it was (for example only once) in the whole history of watch. It will be nice to have an option to determine number of last entries to check in the history (how much in the past we are searching for), or an option to auto clear the history older than X hours/days/checks.

**Describe the use-case and give concrete real-world examples**
I have a watch for HTML elements with label "SALE!" with an option "Only trigger when new lines appear" turned on (content on the page is moving around, so this option is a bless for me). For the first time when this text appears, my watch triggers notification. But at the next sale (after few days/weeks), when text "SALE!" appear again, notification will not be sent, because this text is already in the history.