Updatable/mergeable indices #3

Now that we're using index files as described in #1, there are some features we can additonally implement. One of them is to update an archive.

As I wrote in #1,

Should we ever want to add data to the archive, we can append to the content file, and write a new index.

It might even be useful to merge index files, as there is already a script to do that in fst.