Add support for AWS CLI configuration variable reuse

[sopel]

This has been triggered by #44, see Other Configurable Variables for the respective AWS CLI functionality.

Given botocross is basically an extension of the functionality provided via boto/botocore and the AWS CLI, it seems to make sense to reuse the respective configuration variable functionality to ease every day operation - in particular the profile support (see Getting Started) would allow to maintain separate configurations for botocross, if need be.

[sopel]

At first sight this depends on #47, at least if The botocore Session Object should be reused for configuration variable management, which is obviously the right thing to do.