Allow packaging of any file as a tag file

[diamondap]

Allow DART users to add files to bags as tag files. These files would be packaged in the bag, but they would be put outside the payload (data) directory.

This is a fairly common case, especially with XML metadata files exported by DAMS or internal/institutional repository to be preserved alongside the content payload. Bag structure would look like this:

bagit.txt
bag-info.txt
manifest-sha256.txt
custom-metadata.xml
data
data/file1.pdf
data/file2.pdf
data/file3.pdf

In the case above, user should be able to drag custom-metadate.xml into DART and ask that it be added as a tag file rather than a payload file.