Support passing passing multiple proto files to protoc instead of one by one

[Fadelis]

Currently the plugin executes multiple protoc commands for each proto file. I think it would be much more effective and is required by some protoc plugins (openapiv2) to pass all the proto files as a single command. Not sure if there was a specific reason for current implementation though.
I haven't found a way to achieve this with current plugin, so would be a great addition I think.

[Necropaw]

#119#119

[timw]

This would be required to let https://github.com/pseudomuto/protoc-gen-doc work (same as OpenAPI, most doc output plugins want to produce a single book style output, so require all proto files in one invocation).