Non-destructive Detection of Outdated Entries

[EntilZha]

Hi, thanks for the great tool and data!

For a document I'm working on I wanted to use rebiber to find outdated entries, but since I put a lot of care into curating my bibliography I didn't want to have it overwrite my existing entries. To meet my use case, I added another utility that re-uses the rebiber code/data, but does this instead:

1. Detects arxiv papers
2. Checks to see if existing conference versions exist
3. Prints the original entry and the new entry to the console, so the user can decide how to fix it.

I'm pretty tight on time to work on this much more, but thought this might be helpful to someone else so doing this PR. Eventually I'd like to add more gradual bib editing (e.g., preserve bib entries, but add DOIs).