"strict" option to check IPv4 CIDR network addresses

[ingvaldlorentzen]

Hi,

This PR adds the ability to set the CIDR validation as "strict" fulfilling the wish stated in #6.

It adds a new optional boolean parameter named "strict" that defaults to false, thereby not being a breaking change.

It adds a new dependency ipaddr.js for getting the network address from an arbitrary CIDR address.

The current implementation only supports IPv4.

Any feedback is appreciated!