Generate an openbox preview if possible

Whenever a link is posted to something that openbox can handle (http://dysania.github.io/onebox/), it generates an html preview for it. I tested it out with github commits and gitsts, and it seems to work, but it needs a lot of work to make it look pretty and customize the previews (the gists' "preview" was the whole gist, it needs to be a shorter summary).

Right now, I wouldn't merge the PR, but would first investigate how we can style the previews nicely.