Create packager-specific DSLs

:red_circle: :construction: :construction_worker: 

This PR permits the user to define custom packagers on the project. For example:

```ruby
name 'chef'
# ...

package :msi do
  parameters upgrade_code: 'ABC-123'
end

package :pkg do
  identifier 'com.getchef.chef' # formerly Project#mac_pkg_identifier
  sign true
  signing_identity 'whatevs'
end

package :rpm do
  sign true
  signing_passphrase ENV['SUPER_SECRET_PASSPHRASE']
end
```
@schisamo it was hardcoded, but then the team just overrode the entire template ([see here](https://github.com/opscode/omnibus-chef/blob/0cde29bb1d1fa0a40d22f3c464ac44cdbfef024a/files/chef/windows_msi/Resources/parameters.wxi.erb#L14)). 

I guess technically these examples are incorrect, as it should be `"UpgradeCode"` with the value. ([see here](https://github.com/opscode/omnibus/blob/sethvargo/packager_dsl/resources/msi/parameters.wxi.erb#L6-8)).