Add input and output rpm trigger support

I needed to be able to use rpm triggers and I noticed that fpm didn't support them... I just added the support with rpm-specific options.

It would be nice to be able to support "generic" for package systems that support them (I'm thinking at .deb specifically), but I didn't have time to play with this.
This code is a bit complex and difficult to read. I'll probably merge anyway and we can improve this later.