Add support for analysis of targeted sentiment

[antoine-sachet]

There is a new AWS Comprehend feature: Real-time analysis for targeted sentiment

See https://docs.aws.amazon.com/comprehend/latest/dg/using-api-sync.html#get-started-api-targeted-sentiment

The API is documented here:
https://docs.aws.amazon.com/comprehend/latest/APIReference/API_DetectTargetedSentiment.html