Create xml output

Xml output can now be specified at the command line by providing the -x option.  When Xml output is chosen, the prior labels (Function x or Differential Function x) are omitted, and the output is human readable Xml like the following:

<functions>
  <output>
    f(x) = +35x^66 - 24x^48 - 38x^23 - 4x^14 + 1x^2 
  </output>
</functions>