Provide support for AWS::CloudFormation::CustomResource

[howech]

It would be nice if there were a way to include a way to declare the custom properties and attributes that the custom resource allows, as well.

[howech]

PR #62 adds the basics of a way to build a custom resource, but it does not include a way to explicitly declare custom properties. I haven't tried it yet, but you can probably declare free-form properties somehow....

[gergnz]

Just bumped into this one. It's not obvious, but I'm going to take a look and see what I can work out.

[gergnz]

I was able to create arbitrary values using the Property method:

  CloudFormation_CustomResource('TagElasticCache') do
    ServiceToken "arn:aws:lambda:ap-southeast-2:123456789012:function:myfunction"
    Property("Tags", [{'Key' => 'Name', 'Value' => 'myname'}])
    Property("Arn", "arn:aws:elasticache:ap-southeast-2:123456789012:resourcetype:resourcename")
  end