Add IP Anonymization Contains method

[stephen-soltesz]

This change adds a new method to the anonymize.IPAnonymizer interface:

• Contains(dst, ip net.IP) bool

This new method allows a caller to assess whether a given dst IP "contains" the second given ip, if dst were treated as a netblock based on the configured anonymization method.

For example:

• Does "192.168.0.1/24" contain "192.168.0.3" ? Yes
• Does "2001:668:1f:22::73/64" contain "2001:1900:2100:2d::114" ? No

This functionality is helpful for applying netblock anonymization to hop IPs within a client subnet, even if they are not the same as the client IP.

---

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1202

• 26 of 26 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 94.43%

---

Totals
Change from base Build 1193:	0.06%
Covered Lines:	2153
Relevant Lines:	2280

---

💛 - Coveralls