Add empty element in case of pseudo element symbol in generated button

It creates an empty element in the generated button, dedicated to use pseudo-element content, which won’t be vocalised.

Updated demos: https://van11y.net/downloads/hide-show/demo/index.html
Doc: https://van11y.net/accessible-hide-show/
Announcement: https://van11y.net/2018/01/29/New-feature-for-accessible-hide-show-script-pseudo-element