Add option to format JSON response with an indent.

[hbchai]

This would support human-readable responses, when desired, by setting the runtime.ResponseIndent global. Just the non-streaming response can be indented for now.

[yugui]

Interesting. But can I consider this PR after #53? I guess this feature gets easier to implement without adding hacky flag once we complete #53.

[hbchai]

Sounds reasonable. I think options are probably better in the long run, as long as they are easy to set.

[bufdev]

Generally I see public variables in golang as constants, but there are exceptions - I would generally do something like:

var (
  responseIndent = ""
)

func SetResponseIndent(s string) {
  responseIndent = s
}

But just my opinion.

[dmitshur]

@peter-edge, what would the benefit of having SetResponseIndent be?

[mikeatlas]

@shurcooL the SetResponseIndent could easily do things like strip invalid indentation characters, particularly via a whitelist of simply [" ","\t"], for example. Getter/Setter pattern hides implementation details and can help protect invalid usage of exported variables in a package.

[yugui]

Now we can indent JSON outputs with JSONPb marshaler.