Allow sending -w style to stderr

[paulehoffman]

-w is quite useful, but not to those of us who use curl without -o or -O (such as for scripting through a higher-level language). It would be nice to have an option that is exactly like -w but sends it to stderr instead. Proposed name: --write-stderr.

[bagder]

Thanks for this. Idea added to TODO. Closing this now, hoping someone will take it up.

[jay]

@paulehoffman Related discussion in #1106, I proposed a %{flags:stderr} if you have any feedback on that