Add image preview #70

@FND though you probably don't like #70, could you review the JS? Thanks :)

The idea of this is to add JavaScript code to the page which iterates over all posts and check whether there is text in the post that matches a certain pattern. This pattern would match
1.) A URL prefixed with `!` which is present anywhere in the post
2.) A valid markdown image syntax like `![alternative text](http://my.url/great-image.gif)`
If a matching text is found, the code will append an `<img>` tag to the post to render the (potential) image