Support the AWS CLI's configuration file

This patch adds a feature of configuring AWS credentials and region by *AWS CLI*'s configuration file.

http://docs.aws.amazon.com/cli/latest/userguide/cli-chap-getting-started.html

This feature is implemented by using great *[aws_config](https://github.com/a2ikm/aws_config)* library because of lack of this functionality in *aws-sdk* (upcoming [aws-sdk-core](https://github.com/aws/aws-sdk-core-ruby) V2 will [support it](https://github.com/aws/aws-sdk-ruby/issues/477)).

It also add a `--profile` option to choose one of multiple profiles in the configuration file.