code: function Limiter (options) {
    options = options || {}
  
    this.prefix = options.prefix || 'LIMIT'
    this.max = options.max >= 1 ? Math.floor(options.max) : 2500
    this.duration = options.duration >= 100 ? Math.floor(options.duration) : 3600000
  }
docs: Initialize a new limiter with `options`:
  
  - `db` redis connection instance
  - `prefix` redis key namespace
  - `max` max requests within `duration` [2500]
  - `duration` of limit in milliseconds [3600000]
  
  @param {Object} options
  @api public