add show_cmd_line to Pandoc builder
add show_cmd_line to Pandoc builder

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/rust-pandoc