Additional shortcode attribute for adding custom CSS class(es)

[darrenkearney]

A quick alteration that simply adds an extra argument 'class' to the shortcode and link manager's generated output. I'm not familiar enough with the codebase to know if it needs additional setup etc. but on testing it works a charm.

Now I can have my own custom classes and use several different styles to display relevant posts in a more customizable way. For example:

...post content...
[rp4wp class="style-1"]
...post content...
[rp4wp class="style-2 aligncenter bold dark-background"]
...post content...

This is something I'll be using myself, so I just thought I'd give a pull request to see if it can be added to the next version.

[darrenkearney]

I've just added an offset argument for the shortcode. Going to combine the two enhancements into one pull request.