Feature request: Support for cygwin

I can install and run rsvm in Git Bash, a cygwin Unix-like environment for Windows. However, when I try to use rsvm to install Rust or list Rust versions, I get an error about lack of support:

```console
$ rsvm ls-remote
rsvm: Not support this platform, MINGW64_NT-10.0
```

Could we land cygwin support in rsvm, for Windows users keen on managing multiple installed Rust versions?