Adds allow-empty and locale arguments. Adds global output format option

• Adds allow-empty argument to helpers to prevent Invalid Date to be rendered when passed an empty value (null, undefined, or "")

{{moment-format date allow-empty=true}}

• Adds locale argument to helpers to locally scope locale format to specific helpers

{{moment-format date locale='es'}}

• Adds a global output format string option to config/environment

module.exports = function() {
  return {
    moment: {
      outputFormat: 'L' // overrides the `LLLL` that moment defaults to
    }
  }
};