ENHANCEMENT: Make default email guessing more intelligent

[mroth]

Currently, if kaclone.email if not present in the gitconfig hierarchy, we try to guess default user email by doing $USER + "@" + DEFAULT_EMAIL_DOMAIN.

As @spicyj noted in code review, we could additionally first check if gitconfig has a user.email setting which endswith(DEFAULT_EMAIL_DOMAIN), and if so, use that first. There are a few more places we could check for hints as well, such as the GIT_AUTHOR_EMAIL os.environ variable.