Example Config is IPv4-only, yet tornado supports IPv6

Given this day in age where IPv4 addresses are exhausted, and 20% of the traffic on the internet is IPv6, it would be great to see MotionEye support IPv6 as well.

Looking into the code a bit, it appears that a small change to the settings.py will enable MotionEye to support IPv6 (at least for the control webserver). The current settings.py has:

# the IP address to listen on
# (0.0.0.0 for all interfaces, 127.0.0.1 for localhost)
LISTEN = '0.0.0.0'

Yet, setting LISTEN to an empty string will enable the tornado webserver to listen on both IPv4 AND IPv6

# the IP address to listen on
# (0.0.0.0 or :: for all interfaces, 127.0.0.1 or ::1 for localhost, blank for both IPv4 & IPv6)
LISTEN = ''

At a minimum it would be nice if the comments reflected support for IPv6, and perhaps the default settings.py could be a blank string, enabling both IPv4 AND IPv6 support for your fine software.