gptel.el (gptel-response-filter-functions, gptel-send,
gptel--create-prompt, gptel--transform-response, gptel--convert-org,
gptel--convert-markdown->org): Add support for org-mode by transforming
the response manually. (Note: Asking ChatGPT to format its results in
org-mode markup produces inconsistent results.)
Additionally, the abnormal hook `gptel-resposne-filter-functions' is
added for arbitrary transformations of the response. Its implementation
seems needlessly complex, and in the future we should change it to
use `run-hook-wrapped' with a local accumulator.
