Append to file for extract queries

[CarsonRuns]

When doing an extract, I want to be able to group resources based on logical associations. For instance, I would like to be able to keep AWS::EC2::VPC, AWS::EC2::VPCGatewayAttachment and AWS::EC2::VPCDHCPOptionsAssociation all in one file. One way to achieve this would be to allow appending to an existing file. The extract command would have to be executed three times on the same file name.

[JeroenSoeters]

We are currently in the process of designing a better flow for bulk extraction. I'll create an issue when we have this design fleshed out.