Support a true "full size" option with wysiwyg images

[mshick]

The current behavior is misleading, and limiting. In conjunction with the custom redactor, using the "full size" (internally, in code, called resize_full) icon sends a large command. For those who would like to have an image larger than 1200px in their body, they are left modifying the HTML, and those changes will be wiped out each time somebody uses the wysiwyg image editor.

I will be submitting a PR on the webhook-redactor repo for the complimentary functionality, but will be creating a PR here to support a new imageCommand called full which sets the image resize parameter to s=0, effectively using the natural image size.

The change in this project would be backward compatible, and would only change the behavior for builds using a modified redactor, which will send the full command.