Improve configuration of commands for external postprocessing

[mosbth]

Change how the external commands are defined in the config file.

Now.

        'png_filter_cmd'    => '/usr/local/bin/optipng -q',

Change to

        'png_filter_cmd'    => '/usr/local/bin/optipng -q FILENAME',

It makes it easier to exchange the proposed postprocessing tools to other versions.

Or, make it even more flexible to allow a chain of post processing tools to be executed.