plugin: Allow to declare custom attributes in config files

[wata727]

See also terraform-linters/tflint-plugin-sdk#72

This PR adds Body field with the hcl:",remain" tag to tflint.PluginConfig. This allows you to declare arbitrary custom attributes for each plugin, which previously only allowed enabled as attributes for the "plugin" block.

The content declared inside the "plugin" block will be transferred to the plugin as hcl.Body. On the plugin side, you can retrieve the configurations by decoding it against your own schema.

For example, the AWS ruleset will be able to declare the following configuration:

plugin "aws" {
  enabled = true

  access_key = "AWS_ACCESS_KEY"
  secret_key = "AWS_SECRET_KEY"
  region     = "us-east-1"
  profile    = "AWS_PROFILE"
  shared_credentials_file = "~/.aws/myapp"
}