Allow hide-text addon to accept height argument

- Added the ability to pass a height value (default is `1em`) to the `hide-text` addon, since one is required for it to work.
- Changed the pseudo-element to use two colons instead of one, per [MDN’s note](https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-elements#Notes).
- Fixes #551
I think we should. It can go both ways (single or double colon), so we should pick a style—and in my opinion—the double seems more official: [MDN](https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-elements) does it this way, and we link to them a lot.