Unbreak omnibus and chain packager.

Fixes #69
Add changelog for 0.20.1.
Bump version to 0.20.1.

Changes:

* Unbreak omnibus and chain packagers. (#69)
Add feature to modify fpm object from recipe.

This feature allows recipe to interact with fpm object directly.
Below example shows how you can set the suitable system user name.

class Recipe < FPM::Cookery::Recipe
  platform :redhat do
    fpm_hook do |fpm|
      fpm.attributes[:rpm_user] = 'httpd'
    end
  end

  platform :debian do
    fpm_hook do |fpm|
      fpm.attributes[:deb_user] = 'apache'
    end
  end
end
Added spec for fpm_hook feature.