Perform tone analysis before sending an email

Mail could perform tone analysis on email content before the email is sent. An example of this feature would be:

- Quantify politeness/impoliteness while sending professional emails e.g. job applications, and warn the user in case he has been too rude
- If a user is using an angry tone to answer to a kind request, he could be warned with a message like "are you sure you want to use this tone?"

There already exist several implementations of this kind of systems ([IBM Tone Analyzer](https://tone-analyzer-demo.ng.bluemix.net/?cm_mc_uid=21877007584115340704133&cm_mc_sid_50200000=70964821534070413398&cm_mc_sid_52640000=48781981534070413407)) and there exist several datasets which could suit this problem ([Politeness](https://github.com/sudhof/politeness), [Emotion Detection in Text datasets](https://stackoverflow.com/questions/30703485/data-sets-for-emotion-detection-in-text))

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