Fix for usage with custom storage configs that have options

[cincodenada]

Hello, I am in a project that is using this with a custom storage config, and the current code pulls in the custom BACKEND, but throws away the OPTIONS key from the config, which breaks the config, since the storage doesn't work properly without its options.

This PR remedies that by simply using the sass_processor config if it exists (as with the current code), and falls back to the custom config if it exists, without overwriting the OPTIONS with {}.