Add caption support to all amp-instagram embeds

[hrkhal]

We've started to receive "Social Media Issues" emails from the Google Search Console because our amp-instagram embeds are missing captions which we display on the canonical by default.

They state: "Instagram posts embedded in the article are missing captions, which leads to context being lost."

Caption support for amp-instagram embeds was only added at the end of March this year.
ampproject/amphtml#8243

I propose making the necessary changes to amp-wp/includes/embeds/class-amp-instagram-embed.php to allow captions to be shown as default for all amp-instagram embeds to avoid future "Social Media Issues" for publishers.

If people are happy for this go ahead, I'll knock up the needed PR as soon as I can.

[westonruter]

Makes sense to me. Feel free to open a pull request.