Add support for adding headers in .s3cfg file

Adding headers with --add-header from the command line is helpful  But, it's sometimes nice to be able to set headers for all files in the .s3cfg. For example, to use AES256 encryption on all files uploaded. There are other places this is useful as well.  It's a simple change that I think adds decent functionality to s3cmd.  Here's how it looks in a config file:

[default]
add_headers = x-amz-server-side-encryption:AES256, x-foo:something