Allow generating response programmatically

[kornelski]

Fixes #64

[lipanski]

first of all, thanks for this PR 🥇

a few notes:

1. please check the 1.32 failures in Travis. I'm not against bumping up the minimum supported version but only if there are no (easy) fixes.

2. what happens if we'd like to change the response status programmatically? or the headers? is there any way this could be adressed here? if not, I'm actually fine with your implementation but the interface might change once we bring the status & headers in.

3. I assume you've used the chunked transfer-encoding because you don't have access to the body at all so it's hard to compute the content length? or could we still have an option for the content-length based transfer?

[kornelski]

1. fixed
2. the callback is called too late to change these. It's better to have a separate API for this.
3. Chunked encoding allows the callback to stream the response without buffering. It makes it possible to output some data, flush/wait, then output more.

[lipanski]

@kornelski

1. you've hit the clippy failures now (which is set to 1.32 as well). clippy can be changed to stable if you want but that brings other joys (=errors to fix).

2. ok, fair enough.

3. ok, thanks for the explanation.

[kornelski]

Updated

[lipanski]

Released as 0.19.0 🚀 Thanks!