feat: add versioning flag/arg

Add versioning, you can use ref from [pratikbin/go-nproc](https://github.com/pratikbin/go-nproc/blob/d9c5627c78226990ad2035f7d81b3007020a5f9a/main.go#L9-L11
)

```shell
$ go-nproc -version
go-nproc v0.0.0

Written by Pratik Balar (github: pratikbin)
```