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)

Seeing you other comment below this may be the wrong approach to begin with but using what I was doing this is what I was dealing with. I got it to work using shortcode but it could break depending on how the user input. So this, multiple atts  [gallery ids="32,30" foo="bar" columns="2"]
parses like this
```
array(4) {
  [0]=>
  string(8) "[gallery"
  ["ids"]=>
  string(5) "32,30"
  ["foo"]=>
  string(3) "bar"
  [1]=>
  string(12) "columns="2"]"
}
```
So getting $atts[ids] was easy but then what about columns? It's not keyed.
Then if you do something like this [gallery ids="32,30"] then you would have to get id's a different way.
```
array(2) {
  [0]=>
  string(8) "[gallery"
  [1]=>
  string(12) "ids="32,30"]"
}
```