Refactor ipaddr() filter plugin

ipaddr() filter has been refactored from the group up. Function uses
network representation of an address internally for all checks and
outputs the correct format (IP address or subnet) by examining user
input, if not specified.

Lists are now fully supported and ipaddr() can convert addresses from
and to integers without any issues.

New 'cidr' query has been added which outputs IP addresses and subnets
in CIDR notation (you can for example output '192.168.0.1/24' to
indicate a router), ipaddr() can also read this notation without any
issues. 'subnet' query has been modified and it now outputs
network/prefix in CIDR format, for example '192.168.0.0/24'.

'6to4' query outputs now a host address instead of network address, in
other words, '2002:aaaa:bbbb:cccc::1/48' instead of
'2002:aaaa:bbbb:cccc::/48'. This format covers more useful information,
for example it indicates the router for the network automatically.
Update CHANGELOG.rst
Merge pull request #39 from drybjed/refactor-ipaddr

Refactor ipaddr() filter plugin