[arrow-parens] Add a "when-brace" option

Our team would like to adopt the AirBnb best practice for arrow functions parens but it appears the current options of this rule doesn’t fit it. Since we want to rely on automated coding style rules only and found out it would be useful for others too, here is the PR that fulfill it.

See https://github.com/airbnb/javascript#arrows--one-arg-parens Discussion: https://github.com/SimpliField/eslint-config-simplifield/issues/14

• The version of ESLint you are using

2.11.1 https://github.com/SimpliField/eslint-config-simplifield/blob/master/package.json#L20

• The rule you want to change

‘arrow-parens’

• The code you want to be flagged as incorrect

a => { a } // (1)
(a) => a // (2)

• What happens when the rule is applied to the code without your change

with ‘as-needed’ (1) is valid

with ‘always’ (2) is valid

[kaicataldo]

I can champion this!

[kaicataldo]

Thanks! Have we decided that requireForBlockBody is the config option name we want? Sounds good to me, just want to make sure.