plots diff/show: `--open` flag to directly open it in the default browser

[skshetry]

As the title says

[efiop]

Context #4158 (comment)

[efiop]

Might be easier to just click the link than use that flag, but we might consider doing that by default when in interactive mode. Not sure.

[skshetry]

Using 🖱️ is quite an effort. You have to hover it to the location of the link, and then Ctrl-Click it. Quite a hassle. 😄
And, not everyone's terminal is nice enough to support that.