Add cfg option for opening files outside terminal

Since adding the "open file from transmission-remote-cli" feature, there
has been a commit which made this feature a bit annoying for me since
opening a file would've meant that transmission-remote-cli should stop
until the process running using the selected file would have ended.

Therefore, if 'file_open_in_terminal' is set to False, 'xdg-open <file>'
will start in the background, independent from the terminal, with stderr
and stdout redirected to /dev/null. The 'transmission-remote-cli'
process will not be stopped. By default, this option is disabled (set to
True).