Conditionally solves #2.
gptel.el (gptel-use-curl, gptel-parse-response, gptel--playback,
gptel-send, gptel-playback): New user options `gptel-playback',
`gptel-use-curl`. The former controls whether the response is played
back in chunks, which is done by the function `gptel--playback'. The
response returned by `gptel-get-response' and `gptel--curl-get-response'
is now a plist with the content and status.
gptel-curl.el (gptel--curl-get-args, gptel--curl-get-response,
gptel--curl-sentinel): Add support for curl when available. Set it to
the default. `url-retrieve' is full of fangs that multibyte you.
