Add threadshold option

[jd-solanki]

In mac, there is a similar tool and it provides a threshold option to accurately measure two borders.

e.g. In below screenshot top is calculated from above input rather than from within the current input

[mrflix]

I know this challenge with subtle color changes. I added a fixed threshold to allow for correct measurements around anti-aliased text. I think shortcuts to increase or lower this threshold on per measurement-basis would be a smart solution for cases with subtle color areas. Scroll up/down would be quick but it would disable the page scroll, which currently resets the measurement and takes a new one upon scroll-end. Because of that I think the best solution would be using the + and - keys. And as a visual feedback a toaster notification that quickly disappears.

Are you interested in adding this feature?

[jd-solanki]

I love to make PR but code looks too mathematical 🙄

Moreover, I don't have any experience or expertise in developing the extensions.

Regards.