settings for default caption(title/description) when inserting image

There used to be a setting for "preload caption" in the 4.6 version of img_assist, but it seems to be gone in 4.7. Personally, I prefer description with no title, and I'm sure there are people who have preferences other than the default bolded title and description too.

Here is a simple patch that adds "Preload title", "Preload description" settings to the module.