Awssso

* Credentials are now cached using keyring. When using login, you can provide `-r` option to force credentials renew.

* JSON login option to support [Sourcing Credentials with an External Process](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-sourcing-external.html) with awscli (thanks [ajkerrigan](https://github.com/ajkerrigan))

* Initial release
* Console Sign-In
* Properly detect authentication failures

* Initial pre-release