backport of Nanoc::CLI.after_setup functionality

This commit backport the useful [Nanoc::CLI.after_setup](https://github.com/nanoc/nanoc-cli/blob/master/lib/nanoc/cli.rb#L69) functionality from Nanoc 4.x to Nanoc 3.x

This backport is really useful to initiate the migration from Nanoc 3.x to Nanoc 4.x since developers can already start packaging their plugins following the Nanoc 4.x packaging style!

For instance, the plugin [nanoc-photomator](https://github.com/barraq/nanoc-photomator) is packaged as follow for Nanoc 3.x

```ruby
# encoding: utf-8

$LOAD_PATH.unshift(File.expand_path('../lib/', __FILE__))
require 'nanoc/photomator/version'

Gem::Specification.new do |s|
  s.name = 'nanoc-photomator'
  s.version = Nanoc::Photomator::VERSION
  s.homepage = 'https://github.com/barraq/nanoc-photomator'
  s.summary = 'Photo automation tools for Nanoc'
  s.description = 'Provides a set of commands and user interfaces for working with pictures in Nanoc'

  s.author = 'Rémi Barraquand'
  s.email = 'dev@remibarraquand.com'
  s.license = 'MIT'

  s.required_ruby_version = '>= 1.9.3'

  s.files = Dir['[A-Z]*'] +
      Dir['{lib,test}/**/*'] +
      ['nanoc-photomator.gemspec']
  s.require_paths = ['lib']

  s.rdoc_options = ['--main', 'README.md']
  s.extra_rdoc_files = ['LICENSE', 'README.md']

  # For Nanoc 4.x
  #s.add_runtime_dependency('nanoc-core')
  #s.add_runtime_dependency('nanoc-cli')
  
 # For Nanoc 3.x
  s.add_runtime_dependency('nanoc', '>= 3.6.7', '< 4.0.0')
  

  # Autoloading
  s.add_development_dependency('bundler', '~> 1.5')
end
```