setup Prettier for code formatting

[staltz]

The only change I did by hand was in package.json, all the other files were formatted automatically by Prettier (saving you time from reviewing those files).

Diff for package.json:

https://github.com/ssb-js/ssb-keys/blob/6e430df9972230293d0f63cc4bbd08705ee811bc/package.json

[christianbundy]

Thanks for the patch!