Allow flags to be passed on the end of the command line?

One of the things I find makes ordinary cURL easy to work with is that it allows tacking arguments onto the end of the command line, which in practice tends to mean things that vary per invocation (especially payloads) can live on the end and be edited easily.

I'm aware that `grpcurl` inherits its behaviour from Go's stdlib flag package, but would you be open to allowing flags to be specified at the end too?

I had a poke around and this stack overflow answer offers an approach that should work: https://stackoverflow.com/a/25113485/173062

I'd be happy to send in a PR if you're open to this