New plugin: Body overflow element highlighter

[hchiam]

For #500.

Flags:

onmouseover:

sticky behavior (and icon change):

[hchiam]

To test it out in http://localhost:3000 :

// run this in console:
$("article.artboard.message:nth-of-type(2)").style.width = "135vw";
// run plugin and try scrolling around the page