urlpreview: Ability to modify HTML template

[peterrus]

In my opinion the HTML that is returned by the bot renders in a way that takes up too much space in the chat. In a previous version of the plugin I hacked the following:

If we defer all hardcoded HTML to a simple templating engine users can decide how they want to render previews.

I might give this a try myself sometime.

(The example above was implemented using HTML tables. Hello 1997.)