Handle False values in ipaddr() filter

If false values like False or "" are passed to ipaddr() filter, it will
now correctly return False instead of creating an exception.
Change how ipaddr() filter handles wrong values

Previously ipaddr() emitted an error instead of returning False for all
filter types except 'bool'. Now it will also return False when IP
address types are checked ('ipv4', 'ipv6'). Other checks will emit an
error when value checked is not a correct IP address or netmask.
Add '6to4' query to ipaddr() filter

This query lets you convert IPv4 addresses into IPv6 6to4 prefixes in
2002::/16 subnet (onlu public IPv4 addresses should be used). If you
pass IPv6 address or subnet instead, ipaddr() filter will return the
value or False depending on wether it's in 6to4 address range.
Add split() filter plugin

This plugin lets you split strings into a list using a separator.
Update CHANGELOG.rst
Merge pull request #32 from drybjed/filter-update

Filter update