add -dump flag and Dump() method.

[fujiwara]

This PR adds the -dump option and the tfstate.Dump() method. Dump all resources, outputs, and data sources in tfstate.

$ tfstate-lookup -dump

The output is a JSON object. The keys are the address of the resource, output, or data source. The values are the same as the lookup result.

{
  "aws_vpc.main": {
    "arn": "arn:aws:ec2:ap-northeast-1:123456789012:vpc/vpc-1a2b3c4d",
    "assign_generated_ipv6_cidr_block": false,
    // ...
  },
  "data.aws_ami.foo": {
    "arn": "arn:aws:ec2:ap-northeast-1:123456789012:ami/ami-1a2b3c4d",
    // ...
  },
  "output.foo": "bar",
  // ...
}