Python Classwork #3 (ReverseWordList)
03) Write a Python program that allows the user to enter an
    unlimited number of words (an empty entry indicates that the
    user is done typing words).  Then display the list of words
    back to the user in reverse order (the opposite of the order
    in which they were entered).  Make sure your program removes
    leading and trailing spaces from the user entries and displays
    the words back to the user in a neat manner, one per line.
    This program should not import or use any external functions.


    Example Program Run
    -------------------
    Please enter a word (ENTER by itself to quit):  elephant
    Please enter a word (ENTER by itself to quit):  zebra
    Please enter a word (ENTER by itself to quit):  cat
    Please enter a word (ENTER by itself to quit):  porcupine
    Please enter a word (ENTER by itself to quit):  

    Your words in reverse order:
      porcupine
      cat
      zebra
      elephant


  ADVANCED OPTION FOR CLASSWORK #3
    Display a chart showing, in alphabetical order, the letters
    of the alphabet, along with the number of times that each
    letter (ignoring case) appears in the complete list of words.
    For example, if the list contains only the words "kitten",
    "tiger", and "bear", then, in addition to displaying the
    words in reverse order, the output should display a chart
    looking something like:

    Letter Frequency:
      A --> 1
      B --> 1
      E --> 3
      G --> 1
      I --> 2
      K --> 1
      N --> 1
      R --> 2
      T --> 3

    Display only the letters being used by the words in the list.