Add a linter rule which disallows parens around single argument arrow functions.

[KevinGrandon]

This is the plugin implementation work for: standard/standard#309

[KevinGrandon]

@feross @xjamundx - Hello, StandardJS newbie here. What is the procedure for having this rule be evaluated and landed in a way which will eventually be picked up by the StandardJS project?

[xjamundx]

Great rule, but I think it's covered by:
http://eslint.org/docs/rules/arrow-parens#as-needed

Feel free to ping if you disagree!

As far as getting things added I think you did the right thing and started the conversation in the other repo. Let's see where that leads!

[KevinGrandon]

Good to know about as-needed, thanks! (When I wrote that plugin it didn't exist!)