Allow custom prettier rules

[ebaker]

Regularly using quite different prettier rules on projects, would be great if there was a way to specify them. Might make sense to break them out into prettierrc.json file to make them easier to override potentially? These are the ones i'm regularly using:

     {
        trailingComma: 'es5',
        tabWidth: 2,
        semi: false,
        singleQuote: true,
        useTabs: false,
      },

I also need to change 'comma-dangle': 0 in .eslintrc.js for the above rules to work. Happy to discuss how to handle this 🙂