Support flickr_image tag

Hi. I'm a user of vim-octopress.

I use 'octopress-flickr' plugin for my octopress blog. 'octopress-flickr' uses 'flickr_image' tag and that tag is not supported by vim-octopress. I'd really appreciate that if you merged this pull request.

https://github.com/neilk/octopress-flickr