v3.0.0 - Improving the getting started experience

This major release drops support for older versions of Node, so I can take advantage of some of the new ES7 feature. The first time experience has been improved substantially, by giving helpful error messages for things like malformed config file, issues with SSH, and by showing the progress for cloning a repository (the slowest part of running backport).

• Bump to Node 8 and drop support for Node 6 (#59)
• Better handling of configs with invalid JSON (#60)
• Allow user to abort during conflict resolution (#60)
• Gracefully abort if user does not have proper ssh access to github (#61)
• Progress indicator for cloning a repository (#62)