feature: forceMasqueradeCIDRs

[MrMarvin]

In #198 we discussed some use cases of a way to explicitly masquerade, vs the current non-masquerade approach. This PR is an attempt at implementing this feature.

forceMasqueradeCIDRs

• Add new config param forceMasqueradeCIDRs that can be used exactly like the existing nonMasqueradeCIDRs list of CIDR ranges, but inverse.
• Render iptables rules with explicit -j MASQUERADE early whenever a destination matches.

Example rule output

root@gke-foobar-e4ed6188-wdl7:~# iptables -t nat -n -L IP-MASQ
Chain IP-MASQ (2 references)
target     prot opt source               destination
RETURN     all  --  0.0.0.0/0            169.254.0.0/16       /* ip-masq-agent: local traffic is not subject to MASQUERADE */
MASQUERADE  all  --  0.0.0.0/0            10.8.0.0/16          /* ip-masq-agent: specific outbound traffic is subject to MASQUERADE (must be before nonMasqRules in chain) */ random-fully
RETURN     all  --  0.0.0.0/0            0.0.0.0/0            /* ip-masq-agent: local traffic is not subject to MASQUERADE */
MASQUERADE  all  --  0.0.0.0/0            0.0.0.0/0            /* ip-masq-agent: outbound traffic is subject to MASQUERADE (must be last in chain) */ random-fully

Happy for any input on this PR - please let me know what y'all think! ✌️

Fixes #198

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