Exempt variables prefixed with underscore from no-unused-vars rule

Created by: valscion

This is useful when e.g. using object spread operator to remove only a certain field from the object.

For example, this can be used to ignore a property from React component's this.props:

class OpinionatedDebugger extends React.Component {
  render() {
    const { justIgnoreMe: _unused, ...rest } = this.props;
    return <pre>{ JSON.stringify(rest) }</pre>;
  }
}

This PR was asked by Dan via Twitter when he replied to my tweet about ignoring unused variables with underscore prefix: https://twitter.com/dan_abramov/status/775764929894809602

Test plan:

Write this to any JS file in the project and observe no warnings are shown:

var obj = { a: 1, b: 2 };
var { a: _unused, ...rest } = obj;
console.log(rest);

Then try to change it to this and observe warnings are shown:

var obj = { a: 1, b: 2 };
var { a, ...rest } = obj;
console.log(rest);

## Same approach elsewhere

This approach is similar in nature to what rubocop, the de-facto linter in Ruby, specifies in its default config as well: https://github.com/bbatsov/ruby-style-guide#underscore-unused-vars

Prefix with _ unused block parameters and local variables. It's also acceptable to use just _(although it's a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop and will suppress their unused variable warnings.