Pass kwargs to json.dumps in singer.format_message

[jlujan-omg]

Allow configuring json encoder by passing kwargs through
to json.dumps in singer.format_message. Use case is passing data
from pandas.DataFrame containing NaN to encode as null

[cmerrick]

Hi @jlujan-omg, thanks for your contribution!

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[cmerrick]

You did it @jlujan-omg!

Thank you for signing the Singer Contribution License Agreement.

[kulte]

When will this be merged, @cmerrick? This is very important as currently (as just one example) anything which needs to write a RecordMessage in non-ASCII is not possible.

[abij]

The package version updates are unrelated and therefore should be excluded.

And a test would be nice to proof that is works + better coverage.
A non-ASCII test is useful, setting the json.dumps(..., ensure_ascii=False).
I think this setting should default to False if you ask me.