Add convenience API example to CommandLine class Javadoc

[remkop]

Add convenience API example to CommandLine class Javadoc.

 * <p>
 * Another example that implements {@code Callable} and uses the {@link #call(Callable, String...) CommandLine.call} convenience API to run in a single line of code:
 * </p>
 * <pre>
 *  &#064;Command(description = "Prints the checksum (MD5 by default) of a file to STDOUT.",
 *          name = "checksum", mixinStandardHelpOptions = true, version = "checksum 3.0")
 * class CheckSum implements Callable&lt;Void&gt; {
 * 
 *     &#064;Parameters(index = "0", description = "The file whose checksum to calculate.")
 *     private File file;
 * 
 *     &#064;Option(names = {"-a", "--algorithm"}, description = "MD5, SHA-1, SHA-256, ...")
 *     private String algorithm = "MD5";
 * 
 *     public static void main(String[] args) throws Exception {
 *         // CheckSum implements Callable, so parsing, error handling and handling user
 *         // requests for usage help or version help can be done with one line of code.
 *         CommandLine.call(new CheckSum(), args);
 *     }
 * 
 *     &#064;Override
 *     public Void call() throws Exception {
 *         // your business logic goes here...
 *         byte[] fileContents = Files.readAllBytes(file.toPath());
 *         byte[] digest = MessageDigest.getInstance(algorithm).digest(fileContents);
 *         System.out.println(javax.xml.bind.DatatypeConverter.printHexBinary(digest));
 *         return null;
 *     }
 * }
 * </pre>