Allow settings shared credentials file path

[wata727]

Fixes #85

This PR allows passing a credentials file path as an option. There are two ways:

CLI flags

Added --aws-creds-file in CLI flags.

$ tflint --deep --aws-region us-east-1 --aws-profile default --aws-creds-file ~/.aws/creds

References to home directories are expanded automatically. Note that you need to specify the region and profile name. If not specified, the option is ignored. This issue is fixed by other pull requests.

Config file

Added shared_credentials_file attribute.

config {
  aws_credentials = {
    region                  = "us-east-1"
    profile                 = "default"
    shared_credentials_file = "~/.aws/creds"
  }
}