Adds a -O option to the open command that writes the newly created issue id to a file

Adds a -O option to the open command that writes the newly created issue id to a file, e.g

ghi open -O $(mktemp) ...

I've been using ghi in workflows where I run ghi open as part of a script need to capture the newly created issue id in order to perform subsequent steps in the workflow. When using emacs as $EDITOR, I can pipe ghi open to tee and then parse the issue ID from stdout, but for other editors that don't manipulate the terminal directly, this isn't possible.

It probably makes more sense to get this value from the environment, since the feature is mostly useful for scripting, but I didn't