Add support for initial state

[christianang]

For more feature parity with the s3 resource the initial state params should be supported by this resource.

https://github.com/concourse/s3-resource#initial-state

[christianang]

• Check returns initial_version when no blob exists and using versioned_file
• In creates a file named versioned_file when no blob exists and initial_version is set
• Check returns initial_path when no blob exists and using regexp
• In creates a file named regexp+initial_path when no blob exists and using regexp
• In populates the created file with initial_content_text when provided
• In populates the created file with initial_content_binary when provided
• Update readme with initial state documentation