fix: only use prepared commit message if run as hook

[webdevian]

Problem: The prepared commit message feature added in #122 sometimes sets a default from the COMMIT_EDITMSG file when I run git cz. As this message is from a previous commit it is annoying to see it there. It's not a huge problem in the subject as I will always overwrite a new subject, but in the body when I hit enter to leave the body empty it fills it with the default.

Solution: Only set the default when cz-customizable has been run as a hook or in standalone mode

I couldn't think of a better way to detect whether to turn this feature on but I think this a safe solution and won't break it for anyone that is reliant on it