Pick root directory of archive

This allows choosing which directory in the folder structure should be the root of the archive so that if you have:

	build/
		output/
			file1.js
			file2.jpg

you can do

	gulp.src('build/output/**/*')
	    .pipe(tar('archive.tar', 'build/output'))
	    .pipe(gulp.dest('dist'));

and have `archive.tar` contain only:

	file1.js
	file2.jpg

directly at the root level rather than with the whole local folder hierarchy.