Use of external programs to download external links

You can now specify in the configuration which program should download URLs of a specific domain.

An entry in the configuration looks like this:

"external_file_downloaders" : {
     "bbb.rub.de" : "bbb-dl --add-webcam %U",
     "bbb-cluster.di.unito.it": "curl -L -O %U"
}

Where %U is replaced with the URL of the external link.

If the program terminates with an error, the StdErr is written to the log file. During the execution of the program, the current status line of the program is displayed in the status display of moodle-dl.