code: function mirrorHorizontalPlacement (placement, delimiter) {
    return executeMirrorFunction(placement, (first, second) => {
      return [first, second].map(value => {
        return (value === 'start' || value === 'end') ? mirror[value] : value
      })
    }, delimiter)
  }
docs: Given a string or array of one or two placement values, mirrors the placement
  horizontally.
  
  Examples
  ```js
  mirrorHorizontalPlacement('top start') // input
  ['top', 'end'] // output
  
  mirrorPlacement('top start', ' ') // input
  'top end' //output
  ```
  
  @param {string|Array} placement - a string of the form '<value> <value>' or array [<value>, <value>]
  @param {string} delimiter - when provided, a value with which the result array will be joined
  @returns {string|Array} - an array of values or, if the delimiter was supplied, a string of
  delimiter separated values