Remove xdg-open dependency
For CLI environment, taskopen should not depend on XDG. Setting the FILE_CMD will replace the use of xdg-open with a user-defined setting.

We therefore need a better way to discover what mimetype-based open tools are available on the system. On debian-based systems, there is the `run-mailcap` command that uses the `mailcap` file. A prompt if no known tool could be found (neither xdg-open, nor run-mailcap) might be a good idea.

Note, that `taskopen -V` will still try to execute xdg-open even if FILE_CMD has been set.
