[Enhancement] Link previews for HTML files

[3raxton]

Hey, @cfinke, thank you for all the hard work you've put into OSX-Messages-Exporter; I've been looking for something similar for quite some time and this finally provided what I needed. Thank you!

I was thinking about enhancements and had the idea of adding the link previews in the HTML file that is exported using an href to display the existing URLs in chat. I'm not sure how practical this is considering there could be some issues with previews, but figured I'd make the suggestion. I'd have to imagine a site with their meta tags in order would be able to support this if implemented.

Whether implemented or not, I wanted to express my appreciation for the time put into OSX-Messages-Exporter. Keep up the great work and thanks again! 🎉