takes a command line argument wich is a file
spit out error message if file fails to exist
spit out error message if file not specified.

Open text file.
maintain a taally of how many timees each alpha character
is used, case insensitive
(map)

Output is a file containing an HTML table with one row
for each letter, and three columns: letter, number of times
used, frequency to three sigdigs.


<tr><td>f</td><td>3422</th><th>.024</td></tr>

CharacterFrequency.java

Map<Character, Integer> freq = new TreeMap<>();