Autoformat codebase with prettier

[benjie]

Hi @bcherny; this is a genuine PR unlike #12 😳

To ease contribution and make the code follow a defined style, I'd like to propose adopting prettier for code formatting.

I've configured prettier to follow your current style:

• no semicolons
• single quotes over double quotes
• no trailing commas
• limit to 80 columns

I've then auto-formatted the code with yarn prettier:fix in a separate commit. There are no deliberate code changes, only formatting changes.

This will make adhering to your style-guide a lot easier for contributors, and will result in the codebase being consistently formatted throughout.

[benjie]

To ignore all whitespace when reviewing (much more than ?w=1 or git diff -w); you can follow the tip in this tweet: https://twitter.com/Benjie/status/1035488562538586119

git diff -w  --word-diff-regex=[^[:space:]] upstream/master

[bcherny]

This is great, thanks for the contribution @benjie!