Option to show first sticky before hitting first trigger

[jimjam-slam]

In #139 I noticed that videos are a case where you'd often want the video visible (whether or not it's playing) before you hit the first trigger. Having a block of colour there sometimes sucks, and I think even outside video there are cases where you'd want the first sticky visible upfront.

I think this can be handled with CSS (it's essentially "the first sticky, when the section does not have an active sticky"), so it's mostly a matter of whether we want an opt-out class on our .cr-section (say .hide-first-sticky) or an opt-in one (.show-first-sticky). What do you think, @andrewpbray?

[jimjam-slam]

As you note, @andrewpbray, if classes on stickies were propagated from the QMD to the rendered HTML in the Lua filter, a user could simply add .cr-active to any arbitrary sticky to have it initially shown before the user scrolls into a trigger.

[jimjam-slam]

Turns out this does currently work! However, if you're also using .scale-to-fill, that doesn't kick in until you scroll over.