Add option use-json-names to allow using json_name globally

[mshibuya]

I use this project to keeping a proto definition to a BigQuery table. The proto has many nested messages inside.
I use jsonpb.Marshaler to generate json messages which are imported into the BigQuery table. So I tried to use json_name for BigQuery fields as well, but I found that I need to tag every nested messages referenced in the proto with option (gen_bq_schema.bigquery_opts).json_name = true;.
It's kind of error-prone, since the BigQuery schema generation isn't the only usage of the proto, and there's a possibility that anyone creates a new proto without adding json_name = true.
So I wanted to turn it on globally, but I couldn't find an easy way to do so other than adding a global option. Hence this PR.

I'm new to the project. Any suggestions to improve my change will be appreciated.
Thank you for the maintenance effort.

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