async function refresh( source, target, cb ) {

    const paths = { source, target };

    return FRESH_BUILD || await isSourceNewer( paths )
        ? cb( paths )
        : Promise.resolve();

}
Will execute `cb` if `source` is newer then `target`. Can be used to only build changed files.

_NOTE:_ If the `--fresh` flag is defined, this will always call `cb`

@template T
@param {string} source The source file or directory
@param {string} target The generated output
@param {(paths:{ source:string, target:string })=>Promise<T>} cb Called if `source` is newer.