Add max-host-count Operation

[rmorris1218]

Add a max-host-count operations. This would be especially helpful in assessing CIDR blocks in configuration.

For example:

...
  - id: MAX_HOSTS_EXPOSED_PER_RULE
    message: All security group rules must expose less than 1016 hosts
    severity: FAILURE
    resource: aws_security_group_rule
    assertions:
      - every:
        key: "cidr_blocks"
        expressions:
          - key: "@"
            op: max-host-count
            value: 1016
...