Add option to set write timeout

[link2xt]

This feature was added at some point and #62 clearly has been setting write timeout.

But now I am looking at reqwest 0.12.7 and there are only options to set read timeout, connect timeout and request timeout. This does not allow to e.g. upload a large file via POST request, because in this case I cannot set request timeout in advance as request may take an hour and write operation may get stuck if the server stops reading from the socket for some reason.

Grepping the source code for write_timeout reveals only this test which sets request timeout:

reqwest/tests/timeouts.rs

Line 375 in 88bd9be

fn write_timeout_large_body() {

There is a similar issue #870