Multi-year support

[FXCourel]

🎄 Hello ! Thank you for this great tool !

I am personally interested for multi year support (sharing libraries between years, all in one repo) and I saw in #22 that I am not the only one to want this feature.

I found a way to deal with that, by storing other years files apart from the runner and implement a new command (cargo switch-year <Year>) to move these files. Multi-year benchmarks is supported (the one shown in the readme is the year that is currently set up by the user). However multi year star tracking is not supported.

I tried to produce the cleanest code, but I know this system is not very satisfying as moving and managing folders is rather a workaround than a clean and safe solution. Still, I am happy with what I made and I will use it personally.

So this PR is a proof of concept, it's up to you to decide if you want to accept it (with reworks and improvement if needed, I may have forgotten compatibility issues with some features) or if you don't plan to make anything of this.

cargo fmt and clippy were done.
This PR is backwards compatible, i.e. would function immediately on already existing repos.

Would fix #22

[fspoettel]

Hi @FXCourel, thanks for taking the time to open this PR. I will give this a whirl after this year's aoc completes and see how it interacts with git etc.

I can't promise I'll merge it for the reasons you outlined, but I want to commend it as a very creative solution to the problem. 😄