Working directory option for zip packing

I've added `cwd` option for setting working directory

Usage:

structure:

    /build
      /frontpage
        index.html

gulp config:

    gulp.src('build/frontpage/**')
      .pipe(zip('archive.zip'))
      .pipe(gulp.dest('build'));

And we get archive with `build` directory in root. On first usage I've expected that root directory in archive would be `frontpage`. That's why I used this option

    gulp.src('build/frontpage/**')
      .pipe(zip('archive.zip', 'build'))
      .pipe(gulp.dest('build'));

Also, we can use `options` object for describing stream's options