Editor semantic link

Add semantic microformats class attributes to text editor's link dialog if the text format allows them. With this module it's easier to set this semantic links: reply, repost, like, favorite, tag, mention. as explained in https://indieweb.org/Semantic_Linkbacks.

RSVP feature is done by a dedicated rsvp button that creates a data rsvp as it is explained in https://indieweb.org/rsvp:
<data class="p-rsvp" value="yes">I'll be there!</data>