remove vendored clap, make it a dependency instead.

Hi, @djanowski! 

I was reading cutest's source code and found that clap (https://github.com/soveran/clap) was vendored. I removed the vendored clap in favor of adding a dependency. I'm not sure if this is ok to you, but I thought that this would be nice, since the vendored clap might be outdated and the original clap is tested (not tested in cutest).

Hope this helps!