Add Tag plugin link for 'Jekyll Twitter Plugin'

Hey,
I've created a new tag plugin to render Tweets using oEmbed format via the Twitter API so this PR just adds it to the list in the plugins documentation.

It is a slight bit of duplication to the [Tweet Tag](https://github.com/scottwb/jekyll-tweet-tag) (above in the list) but that plugin is currently broken since Twitter forced SSL for this API endpoint and doesn't seem to be maintained anymore. This plugin uses the Twitter gem so should be a bit more resilient to any Twitter API changes and is a gem so easier to use in Jekyll sites.