This is beneficial because it makes grpcurl more like curl. Many more
flags are now common between grpcurl and curl which will help users that
are new to the tool.
* support go modules
- use Go 1.11 (with and without module support) in CI
- add go.mod and go.sum files
- with modules in Go 1.11, can't rely on GOPATH directory layout, so just copy test.proto into this repo