Accessibility : add a span in button.expandmore__button

• To avoid ::before or ::after content directly added to the button (may be read by some AT), thanks to @AnneCav
• Namespaced previous point :)
• Enhanced demos, website and styles

New option to collapse/expand all items

• added option to collapse/expand all items

Linted, re-indented, missing vars and ===

Cleanup for easters :)

V1.5.0

• Added type="button" to buttons => avoid problems in Sharepoint and in forms
• Removed unecessary selectors

1st release of plugin

As kindly suggested in #19 ;)