Recursive directory creation for target

When copying a single file or nested directory, copy the asset recursively. For example:
    src: asset/media/css/styles.css
    dest: media/css/styles.css

Would fail, because media/css destination directory doesn't exist. Specifying a new `createDest: true` option, will create the required rectories recursively. `createDest` defaults to false, to preserve the original behaviour.