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.
The addition of this code has rendered general RPM input non-functional ...

```ruby
val = tags[:triggerindex].zip(tags[:triggername],tags[:triggerflags],tags[:triggerversion]).group_by{ |x| x[0]}
```

This line raises the following error when attempting to input a localized RPM:

```
undefined method `zip' for nil:NilClass
```