Make STDOUT flushing configurable in udp_bridge

See #155.
To enable, create a flush_delay() function in etc/udp_bridge_conf.py
that returns an integer representing seconds (defaults to 60s).
A flush_delay <= 0 will be near to per-packet flushing of STDOUT.
Merge pull request #174 from hynd/udp_bridge_flush

Make per-line flushing configurable in udp_bridge