Use the first token of --chruby as the default name

[k0kubun]

What's this?

I use ./run_benchmarks.rb --chruby 'before::before --yjit;after::after --yjit' to compare two revisions. I use this so often that I want to make it a bit shorter.

This PR allows you to use --chruby 'before --yjit;after --yjit' while keeping the names before and after, not before --yjit and after --yjit as the current implementation would do.

Why this change is safe

When you compare the interpreter and YJIT, you simply don't need to use --chruby in the first place. So, when you use --chruby, you most likely specify different chruby binaries. Therefore it makes sense to use the first token of a --chruby option as the name of the binary in most cases.

Even if you want to specify the same chruby binary with different options in --chruby for whatever reason, you can still explicitly write name:: before specifying the chruby name and opptions.

Also, I didn't update README.md because use cases written there work as is, and the use of --chruby is already explained well. This change is fairly backward-compatible in that sense too.