Add additional transcription language support for the async API client

[amikofalvy]

As a rev.ai golang sdk user, I would like to be able to submit transcription for languages other than English. Add support for the new language parameter as documented on page: https://www.rev.ai/docs#operation/SubmitTranscriptionJob

New Languages supported at this time are English, Spanish, Portuguese, French and German. Language parameter is provided as a ISO 639-1 2-letter language code.

This issue is being filed in an effort to bring the golang sdk into feature parity with the async transcription api.

[vinoo999]

Is this in progress at all by chance? Could be interested in helping out :)
There is also one more new field released that would be interesting to incorporate related to transcriber options: https://www.rev.ai/blog/rev-improves-accuracy-by-over-25-with-launch-of-new-v2-asr-model/
Perhaps worthwhile adding some *obj field to encapsulate any future fields rev incorporates