add CidrLimit configuration variable

[stonith]

• default to 64 if CidrLimit configuration variable doesn't exist
• Allow setting CidrLimit limit to be set via configuration variable

supercedes #91

[k8s-ci-robot]

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In response to [this](https://github.com/kubernetes-sigs/ip-masq-agent/pull/93#pullrequestreview-1207135447): >Thanks @stonith ! I like this approach better to allow for more flexibility. > >One minor request - could you update also https://github.com/kubernetes-sigs/ip-masq-agent/blob/master/agent-config/config to show case this new field? Thanks! > >/cc @jingyuanliang Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[stonith]

Thanks @stonith ! I like this approach better to allow for more flexibility.

One minor request - could you update also https://github.com/kubernetes-sigs/ip-masq-agent/blob/master/agent-config/config to show case this new field? Thanks!

/cc @jingyuanliang

Done, also updated the README.

[k8s-ci-robot]

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