Add twitter-feed include

[kingthorin]

Following review of @securestep9's video: https://www.youtube.com/channel/UCE2nt-oqRjwEAPSBtCtNVSw it seemed like it would make sense for us to have an include to be used for Twitter Feeds.

Usage like: {% include twitter-feed.html twitter="OWASPLondon" %}

To replace: https://github.com/kingthorin/www-chapter-london/edit/master/leaders.md

### Twitter Feed

<a class="twitter-timeline" data-width="100%" data-height="600"
data-theme="light"
href="https://twitter.com/OWASPLondon?ref_src=twsrc%5Etfw">Tweets by
OWASP London</a> <script async
src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

This has the benefit of making it easier for other projects/chapters to adopt, having a single point for maintenance, not having html in markdown files.

Signed-off-by: kingthorin kingthorin@users.noreply.github.com

[kingthorin]

Ping @hblankenship 😀