Simplify config

- Because the task only ever tolerates a single input file and a single output directory, no need to use a multitask with complex file patterns, just pass the `src` and `dest` as options.
- With pre-determined `src` and `dest` properties in the configuration (instead of different file formats), we can have `cdndeps.options.src` and `cdndeps.options.dest` as default config variables for `cdn_paths`.