add show_cmd_line to Pandoc builder

[humenda]

The function set_show_cmdline allows enabling / disabling the output of the command line arguments to pandoc on stdout. It is disabled by default. This becomes handy when using Pandoc in a subprocess which does a lot of Pandoc calls and where the library output would clutter the terminal.

[oli-obk]

sorry about forgetting about this, I seem to have managed to unwatch my own repository