Add option to retain default config

[nirvdrum]

I'm looking to load up babel-plugin-relay in my CRA app. This should be an additive plugin, not something that replaces the defaults. While I can add all the default entries to my .babelrc, that runs the risk of the entries growing stale as CRA grows. As a result, I think it'd be helpful to have an option that doesn't simply replace the original Babel config, but rather allows prepending/appending to it.