Allow callback on stderr even if there's no actual error

There are cases where the stderr stream provides useful output even though there is no erroneous return code. Such is the case with a "git push" command, where status information from the command is useful, and only output to stderr: http://git.661346.n2.nabble.com/git-push-output-goes-into-stderr-tp6758028p6769629.html

This pull request changes the stderr to behave more like stdout: setting it to true, will write it to log, whereas providing a function does the same as stdout. Another alternative would be to write stderr into process.stderr if config === true...