Added numbering to the filenames of the attachments that are extracted

[bennet3]

Change Description: Added numbering to the filenames of the attachments that are extracted. I needed the attachments to be numbered so that I can see the order that the screenshots happened for UI testing and debugging. Test Plan/Testing Performed: Verified that the numbering of the attachments works by running on several .xcresult files from my own project

[rpendleton]

This will result in different filenames, so depending on the release cycle of the project, it may be considered a breaking change. It might be good to disable the indexes by default and enable them with a command line argument, or save this change until a major semver release.

[abotkin-cpi]

Assigning to myself. We're looking at adding a feature in order to created animated PNGs from the test failure screenshots & to do that, we'll need to create the ordered screenshots feature. Additionally, to support both that & this PR, xcparse has to be re-worked to do a DFS rather than BFS of the tree for attachments/screenshots. While this current PR will look like it's in order most of the time, it can be out of order due to the BFS for attachments rather than DFS.