Add current step that is in progress

[balloob]

Breaking change

Proposed change

This adds the step that the flow in progress is currently on.

Needed to implement home-assistant/architecture#377

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# Example configuration.yaml

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[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as its been labeled with a integration (config) you are listed as a codeowner for? Thanks!
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[elupus]

Would be nice if the post command from frontend also included the step and was verified. I've had situation where frontend and backends state got out of sync, and post data was given to wrong step handler. Only happened during development mind you so might not be a big issue.

Also allowing GET request returning cached current step response would simplify frontend in some cases I think.