Add configuration option for file viewer

This adds a config option to set the file viewer to: 'command %s' instead of a hard-coded xdg-viewer, which is retained as default. subprocess.call is used instead of .Popen to allow interactive commands, and the screen refreshes upon command exit The config UI's width is now variable depending on what's set, within sane limits. Examples:
xdg-viewer %s 
elinks -no-connect 1 %s
/usr/bin/less %s

Pipes and such won't work since this doesn't spawn a shell.