Perl Quiz-of-the-Week #12

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You'll write functions for displaying histograms (bar charts).
We'll use these to display the output from next week's quiz.

You're build one function, 'histogram()', whose arguments will be a
list of numbers.  The function should construct a bar chart and then
return a list of strings which, if printed, would display the numbers
suitably.  

For example, 

        histogram(1, 4, 2, 8, 5, 7)

might return the following list of strings:

        "   *\n", 
        "   * *\n", 
        "   * *\n", 
        "   ***\n", 
        " * ***\n", 
        " * ***\n", 
        " *****\n", 
        "******\n"

when printed, these strings look like this:

           *
           * *
           * *
           ***
         * ***
         * ***
         *****
        ******

The behavior of 'histogram' will be controlled by several global
variables.  $HISTOGRAM_WIDTH and $HISTOGRAM_HEIGHT will specify the
width and height of the result.  The output should be scaled to fit in
a rectangle with $HISTOGRAM_HEIGHT rows and $HISTOGRAM_WIDTH rows.

The bars themselves will be made up of the character $HISTOGRAM_CHAR,
which must be a string of length 1.  

If any of the $HISTOGRAM_ variables are undefined, the function should
use reasonable defaults.