Add ability to extend an existing config with missing defaults

[axos88]

Having an old configuration file, I would like the ability to add any new, missing configuration values after upgrading rustfmt - probably at the end -, to be reviewed and configure according to our style. I would expect this to be be a "maximal" configuration containing all supported configuration values.

Currently rustfmt print overwrites the file with the default config, and needs to be merged by hand - also I'm not entirely sure if the "default" configuration is exhaustive or not.