add a browse-at-least-every-${periodicity} mode

[maxlath]

Concrete example:
1/ I set a time capsule on a URL to re-open it every 2 days
2/ one day later, I browse that same URL
3/ another day later, in the current setup, the URL would be re-opened
this feature request would be to add a mode in which, the 2 days period is reset at step 2, meaning that the URL would be re-opened 3 days after it was initially requested

This would allow to express the desire that "I want to browse this URL at least every 2 days, but not more as that would get spammy"

[gherghel]

Would be great.