Add a codecov badge to this repository, and analyze the commits with codecov

[ollie-iterators]

I noticed that this repository has a way to check the code coverage on codecov: https://app.codecov.io/gh/internetarchive/iaux-search-service, but I don't see a codecov badge on this repository.

It would also be good if the commits were analyzed with codecov for this repository, like the others.

[latonv]

Thanks for pointing this out! I've added the badge and updated the default branch on codecov so that it should properly show commit coverage now.