Add function for enabling backports on ubuntu

[pepijndevos]

Recent Ubuntu releases have backports enabled by default, but with a low priority or something like that.

So to install from backports you need to say {{pkg}}/{{dist}}-backports for the package and all it's dependencies.

This is verbose and tighly couples your code to a specific version of Ubuntu.

Maybe the existing debian backports can be made to do "the right thing" for a variety of systems that have the concept of backports?

[hugoduncan]

We should also support the / (for distribution or archive name) and = (for package version) directly in the package action.