Support MFA token from AWS config file

[radeksimko]

There's already a support for the MFA token through ENV variables, so this simple change is adding support for the same from config files.

Have a look to http://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs
just to confirm correctness of variable name according to Amazon's practices.

[gnethercutt]

#208 already out there and waiting...

[radeksimko]

@gnethercutt Ah, sorry for being blind here! 🤦

btw. AFAIK Mitchell & Hashicorp are now mostly focusing on adopting the official AWS library: https://github.com/hashicorp/aws-sdk-go

which is IMO why it's relatively quiet here.

[gnethercutt]

No worries, it was mostly a shameless attempt at a bump without calling it that. :)