Add optional AWS profile authentication

[JamieCressey]

Adding additional argument (-P, --profile) to allow authentication for AWS profiles (i.e. ~/.aws/credentials)

[geekpete]

This would be great!

[ScottyMJacobson]

A bit late on this, but I feel like the auth process here reinvents boto's wheel quite a bit (and precludes using IAM roles). Per Amazon's Docs, the AWS SDKs recommend this hierarchy when it comes to authenticating a session:

Loaded from IAM roles for Amazon EC2 (if running on EC2),
Loaded from the shared credentials file (~/.aws/credentials),
Loaded from environment variables,
Loaded from a JSON file on disk,
Hardcoded in your application

The first three are actually automatically implemented in boto; since s4cmd is built on boto, they should work implicitly. I'm gonna look into this and see if we can use boto's authentication (and handle the exception appropriately if it fails) if .s3config doesn't exist, that way we can get rid of s4cmd's redundant check for environment variables.

[chouhanyang]

Would there be the same thing in boto3 as we moved along from old boto library?