Add command option to allow passing Go package option.

[jasonfriedland]

Compiling a proto definition without a go_package option now generates the following warning:

2020/11/12 15:09:19 WARNING: Missing 'go_package' option in "pkg/protobuf/api/api.proto",
please specify it with the full Go package path as
a future release of protoc-gen-go will require this be specified.
See https://developers.google.com/protocol-buffers/docs/reference/go-generated#package for more information.

This change allows the user to pass -go-pkg and have it used as the value for option go_package in the generated .proto file.

Tested with protoc version 3.13.0.

[dreisel]

this will be really nice to have...