Gallery radio buttons

A stab at adding the option to override the default [gallery] tag behavior which displays all images uploaded/attached to the post. The user can check "Use alternate image id's" and pass comma separated ids of any images to override the default [gallery] behavior. This is using the WordPress shortcode_atts_{gallery} filter to manipulate the gallery output.
![screen shot 2014-08-22 at 2 31 32 pm](https://cloud.githubusercontent.com/assets/1243118/4017033/9af58ecc-2a3b-11e4-9230-c7c602b11916.png)

Oh jeez I see.... ugh. I totally looked at that and was thinking too much about it being used IN the template file itself, filter went right by me. Hey learned about that shortcode filter though so it's still a win!