Use different objects for S3 and gist credentials in the .copy-pastarc file

[jutkko]

Right now it is something like

my-target:
  name: ""
  backend: gist
  accesskey: ""
  secretaccesskey: ""
  bucketname: ""
  endpoint: ""
  location: ""
  gisttoken: my-token
  gistID: myGistID

But have something that's folded will be more readable.