implement listen options to dump requests or run arbitrary commands

this MR implements various options for the listen command to customize the processing. namely the following three options are implemented (also described in #54 (closed))

  --dump-to LISTEN_CONFIG.DUMP_TO
                        Instead of processing the request, dump it as a file to the given location. If you need further customization, use ``--dump-tool``.
  --dump-tool LISTEN_CONFIG.DUMP_TOOL
                        Instead of using ``--dump-to``, use this option to run a specific command for each request. We will first create a temporary file and then run this command as subprocess. This parameter requires `--dump-to` and two curly brackets (``{}``) in the
                        argument that specify where to insert the target path. Or use ``{path}`` or ``{basename}`` or ``{directory}`` for more explicit control in your command. If you want to process the dumped file further, combine this option with ``--cmd`` **Examples** Copy
                        the request to a given location via rsync:: --dump-tool 'rsync {} .' Copy the request via SSH to another server:: --dump-tool 'scp {} user@machine:/some/folder/' Print the request to stdout and delete the temporary file:: --dump-tool 'cat {path} && rm
                        {path}'
  -c LISTEN_CONFIG.CMD, --cmd LISTEN_CONFIG.CMD
                        Instead of processing the request here, dump the request as file to the disc and run the dedicated command. The specified command must contain two curly braces (``{}``) that will be replaced with the path or basename of th file. Or use ``{path}``, or
                        ``{basename}`` or ``{directory}`` for more explicit control in your command. **Examples** Cat the request (i.e. always return the input to the sender):: --cmd 'cat {}' Copy the file via scp and run some command to process it on a remote machine::
                        --dump-tool 'scp {} user@machine:/some/folder/' --cmd 'some-command /some/folder/{basename}'

closes #54 (closed)