Added support for origin for the Release file.

Added the Origin as a command line switch for the Release file, according to the docs Origin is a key/value pair in the Release file. https://wiki.debian.org/RepositoryFormat#A.22Release.22_files

so you can now do 

```deb-s3 --origin=my-origin```

I needed to add this so unattended upgrades can work with custom repos.