Load images_dir to sprockets paths by default

Unless this has been intentionally left out for a reason, the `images` directory should be loaded by default into the Middleman sprockets environment.

This also adds an `images` paths that might exist in the [try_paths directories](https://github.com/jordanandree/middleman-sprockets/blob/b447a2dfdc9441e720740de53576513e61ee41a9/lib/middleman-sprockets/extension.rb#L33-L39)