Add option to only show events if they are within X minutes

[howardjohn]

It would be great to have a flag to only show unstarted events that are up to X minutes way. This would be the pair to --hide-event-after. For example, could call this --show-event-before

I would expect something like this:
--show-event-before=15m --hide-event-after=15m to start showing an event 15m before it begins, and then remove it 15m after it starts.

This is useful to not crowd the bar when there are no nearby events. It can also serve as a 'notification' that I should be in a meeting now or soon.

I would be willing to send a PR for this if desired.