Use config package to store credentials

[leeper]

From @chrowe via https://github.com/cloudyr/aws.s3/issues/240

Please specify whether your issue is about:

• [ ] a possible bug
• [x] a question about package functionality
• [x] a suggested code or documentation change, improvement to the code, or feature request

Is it possible to retrieve S3 credentials using the config package instead of environment variables without have to specify them every time?

Put your code here: I know that I can set them inline like this... config.yml

default:
  s3:
    bucket: 'my_bucket'
    key: YOUR_AWS_ACCESS_KEY
    secret: YOUR_AWS_SECRET_ACCESS_KEY

R code

s3conf <- config::get("s3")
get_bucket(
  bucket = s3conf$bucket,
  key =  s3conf$key,
  secret =  s3conf$secret
) 

But it would be nice to be able to just set that once like you can with Sys.setenv

[vspinu]

I think the default .aws/config should be used instead of inventing custom configurations options. #53