Added the option to download the code stored in a notebook

[damianoazzolini]

I've added a button to download the prolog code of a notebook in a file called program.pl

[JanWielemaker]

Thanks. Looks useful, but I think it is a little broken. See comment.

[damianoazzolini]

Do you have an example where the code does not work as expected?

[JanWielemaker]

Not directly, but fetching the rendered code seems asking for trouble. The CodeMirror editor works from a pure JavaScript object representation of the text. Getting the rendered text may both get too much and too little. Having to delete leading line numbers is already an indication.