Relicense to release under public domain

This moves the project to [CC0](https://creativecommons.org/publicdomain/zero/1.0/) instead of the GPLv3. I'd like it to be very easy for code to flow back and forth between this and the underlying IG scrapers. I'm also finding that some of this code may be nicely steal-able for other (non-IG-related) work at my day job, for which public domain status is very handy.