Adds output-dir and output-item-list for copying only certain files.

This provides the ability to run merge_target_files without the end goal
of a target files zip. This is useful for users that only want the IMAGES
folder, for example.

Bug: 130304869
Test: python -m unittest test_merge_target_files
Change-Id: If0412b8e1eb85fe09d7b689fd7f56ce84067faea