feat: add option for redirecting info

[dwilding]

This PR adds an option --return-dir for use when cloning a repo. If this option is specified, gimmegit outputs all info lines to stderr and outputs the clone directory to stdout. Otherwise, gimmegit outputs all info lines to stdout, as before.

The intended usage is:

clone_dir="$(gimmegit --return-dir owner/project)"
cd "$clone_dir"

I'm also fixing a bug in the detection of whether to output colors (and other formatting). Previously, gimmegit would only consider whether stdout was a terminal. This meant that errors and warnings wouldn't use color if stderr was a terminal but stdout wasn't.