Support nanoc environments

Introduce basic support for nanoc environment:
- nanoc command is updated to include --env argument
- config are overriden using the active environment if any
- unless define in environment, output_dir is {{output_dir}}/{{env_name}}

Setting environments is done in nanoc.yaml using the `environments` property.
Example usage is:

```
output_dir: output

environments:
  default: &default
    base_url: ...
    ...
  development:
    <<: *default
    base_url: ...
  production:
    <<: *default
    base_url: ...
  yet_another_env:
    <<: *default
    base_url: ...
    output_dir: build
```

Selecting working environment can be done:
- using environment variable `NANOC_ENVIRONMENT`
- using `nanoc --env=[<value>]`

lib/nanoc/base/repos/site_loader.rb

@@ -39,6 +39,17 @@ def site_from_config(config)
         end
       end
 
+      if config.key? :environments
+        env_name = ENV.fetch 'NANOC_ENVIRONMENT', Nanoc::CLI.env
+        env_config = config[:environments][env_name.to_sym] || {}
+
+        # Handle specific properties
+        env_config[:output_dir] ||= File.join(config[:output_dir].to_s, env_name.to_s)
+
+        # Override config with environment
+        config.update env_config
+      end
+
       Nanoc::Int::Site.new(
         config: config,
         code_snippets: code_snippets,

lib/nanoc/cli.rb

@@ -42,6 +42,20 @@ def self.debug=(boolean)
     @debug = boolean
   end
 
+  # @params [string] environment
+  #
+  # @return [void]
+  def self.env=(environment)
+    @environment = environment
+  end
+
+  # Return environment
+  #
+  # @return [string]
+  def self.env
+    return @environment || :default
+  end
+
   # Invokes the Nanoc command-line tool with the given arguments.
   #
   # @param [Array<String>] args An array of command-line arguments

lib/nanoc/cli/commands/nanoc.rb

@@ -10,6 +10,10 @@
   Nanoc::CLI.debug = true
 end
 
+opt :e, :env, 'set environment', argument: :optional do |value|
+  Nanoc::CLI.env = value || :default
+end
+
 opt :h, :help, 'show the help message and quit' do |_value, cmd|
   puts cmd.help
   exit 0