Shortcode

I've added a simple short code for wp-flickr-press. This short code solved two problems that I was having:

1. Linking to static images, which I would later edit on Flickr and so would go missing.
2. Quickly including Flickr photos.

I'd like to add a few features to the short code. Future features include:

- Use the Flickr image title as the shortcode-style caption for the image.
- Ensure that the short code respects all of the wp-flickr-press settings
- Make a function to pre-parse the short code so that the short code doesn't need to repeatedly call the flickr API.

I know this implementation is not great without those three things, but I think the short code may be useful for others.