cfg option for skipping browser open on reload

Somewhat similar to existing `NO_OPEN_BROWSER` option, but I'm imagining a world where the tab is opened once when you run `webdiff`, but subsequent updates are inherited by that page, or can be picked up by refreshing it.

Basically, the reasonable numbers of tabs for one `webdiff` process to open are `0`, `1`, and "many", and this allows for the `1` option.