better markup for jf2>mf2 ashtml p-rsvp values though data elements

[tantek]

jf2>mf2 ashtml p-rsvp values could be marked up better (more flexibly for styling / localization) with data elements

For example on: http://stream.thatmustbe.us/?url=https%3A%2F%2Fwebmention.io%2Fapi%2Flinks.jf2%3Ftarget%3Dhttp%3A%2F%2Ftantek.com%2F2017%2F183%2Fe1%2Fsunday-yoga-presidio+&op=jf2-mf2&ashtml=1&style=img{height%3A44px}

<span class="p-name">is attending.</span>
<span class="p-rsvp">RSVPvalue</span>

could be better marked up as:

<span class="p-name">is attending.</span>
<data class="p-rsvp" value="RSVPvalue"></data>

Or if you're willing to do some per-value (case-insensitive BTW) processing of RSVP properties, this is a bit more desirable due to default visibility.

<data class="p-rsvp" value="yes">going</data>
<data class="p-rsvp" value="no">not going</data>
<data class="p-rsvp" value="maybe">might go</data>
<data class="p-rsvp" value="interested">interested</data>

(yes even with the same apparent text in that last example, worth keeping the value vs innerText there for potential localization of the innerText value)

[tantek]

The linked example currently returns bad p-rsvp values due to aaronpk/webmention.io#80 (so that's a dependency for testing this specific example)