code: function TfIdf( documents, language ) {
    
    if( !(this instanceof TfIdf) ) {
      return new TfIdf( documents, language )
    }
    
    this.language = language || 'en'
    this.documents = this.map( documents )
    
  }
docs: [TfIdf description]
  @param {Array} documents
  @param {String} language