allow empty title/composer/interpret

Closes https://github.com/iem-projects/striem/issues/15
check whether self.streamer is set before using it
return 'None' if config lacks section/option
interface to query any config-value from streamer
allow quitting in config
query allow-quit property from config

if not set explicitely

Closes https://github.com/iem-projects/striem/issues/16
do not set 'closeable' explicitely

so the config-value gets used