Promise the full contents of a file: Files promise content attribute

I think this would be a great addition to make files promises easier for beginners, and more advanced users as well. You can achieve the same with templates today, but it is not intuitive.

bundle agent main
{
  files:
    "/tmp/hello"
      content => "Hello, world!\n";
}

If https://tracker.mender.io/browse/CFE-3269 is resolved, it would also be created by default.