support on_data callback

[pschrammel]

To e.g. downstream a file in falcon it would be cool if the adapter would support on_data callbacks. See https://lostisland.github.io/faraday/usage/streaming

[korbin]

angellist@c47d7ea

is how it can be done. I don't have time to add the appropriate test coverage right now to PR this, but it works.

[ioquatix]

I've released this in v0.16.0 - thanks everyone!