Proposal: Streamline CLI flags and args

As `grpcurl` is using standard `flags` package, it's requires to pass flags before positional arguments.

It pretty difficult to "play around" with different commands. I would propose to use come well established CLI libraries for streamlining this process.

Here are one of the libraries that may work:

https://github.com/spf13/cobra/
https://github.com/alecthomas/kingpin
https://github.com/urfave/cli

The change could be done in a backward compatible way I believe.

I could work on a PR if this change would be desired.