Add embed support for Twitter timelines via new amp-twitter attributes

[felixarntz]

amp-twitter now supports arbitrary data attributes for Twitter timelines, based on a required data-timeline-source-type attribute. All other arguments to pass along to the Twitter script must be provided as individual data attributes prefixed with "timeline-". As an example, if you set data-timeline-source-type to "profile", you also need to pass a screenName argument, which would need to happen via data-timeline-screen-name.

This PR adds support for Twitter timelines, by automatically parsing embed URLs into appropriate amp-twitter elements. The code supports profile, likes and list source types.

For reference:

• https://www.ampproject.org/docs/reference/components/amp-twitter
• https://developer.twitter.com/en/docs/twitter-for-websites/javascript-api/guides/scripting-factory-functions

[felixarntz]

When testing the functionality, parsing into the correct tag works, however on the frontend the corresponding iframes only render as blank. I'd need to investigate why this happens, but maybe someone with more AMP experience has a quick idea about it.

[westonruter]

@felixarntz Is your testing site in HTTPS? AMP requires HTTPS for amp-iframe to render:

An amp-iframe must only request resources via HTTPS, from a data-URI, or via the srcdoc attribute.

https://www.ampproject.org/docs/reference/components/amp-iframe

[felixarntz]

@westonruter Ah yes, that was it. I should really setup SSL for my VVV sites.