Adding shell evaluation to ppt format.

The following code add the following feature:

If a line starts with $$ cmd or $% cmd , it will execute cmd in the shell and append the stdout of the presentation. Additionally, if the line started with $%, it will append `%` at the beginning of every line (useful if used with ---beginshelloutput)

Updated the README with this functionality and added the respective example: examples/shell_eval.tpp

Please let me know if you have some observations or questions.

Regards

Gabriel