Add support for running a command at X interval

[redsigma]

Every RSS feed has an option to Automatically Update Every X hours/minutes/seconds

It would be nice to have this feature for running a custom command.

For example the custom command could be python <absolute_path>/file.py and the output of this script must be a valid .xml output string that will be treated as a local .xml file (probably the output of the script will be cached to a temporary file).

This feature is similar with this, except that instead of having a separate app that generates these local .xml files , you let QuiteRSS do it via the Update at X hours/minutes/seconds feature.

I believe this would be an useful and powerful feature to have.