Add new filter, ipsubnet()

This filter allows you to manipulate IPv4 and IPv6 subnets:

- given a subnet and a CIDR prefix, you can check how many subnets can
  be created from specified subnet;

- given a subnet, a CIDR prefix and an index number, you can get a new
  subnet with specified CIDR;

- given an address and CIDR prefix, you can check the biggest subnet
  that contains given address;

- given an address, CIDR prefix and index number you can get smaller and
  smaller subnets containing given IP address;
Update CHANGELOG.rst
Merge pull request #35 from drybjed/ipsubnet-filter

Add ipsubnet() filter plugin