Add support for Conditions on Resources

[smithhannahm]

Could a similar fix to #132 be implemented to associate Conditions with Resources? I tried a workaround to extend individual resources, like so:

type AWSEC2EIP struct {
	resources.AWSEC2EIP
	Condition string
}

func (r *AWSEC2EIP) MarshalJSON() ([]byte, error) {
	type Properties resources.AWSEC2EIP
	return json.Marshal(&struct {
		Type       string
		Properties Properties
		Condition  string
	}{
		Type:       r.AWSEC2EIP.AWSCloudFormationType(),
		Properties: (Properties)(r.AWSEC2EIP),
		Condition:  r.Condition,
	})
}

But some of the default processing on "Condition" nodes cause resources generated by this workaround to evaluate to 'false', which is not valid CloudFormation.

Resources:
    MyEIP: false