Refactor CodeOcean::Config class

The new architecture memorizes settings (which we mostly did after reading the config so far) and also exposes the resulting file path as well as further settings.

This change is a prerequisite to define a dependency with Sprockets.

lib/code_ocean/config.rb

@@ -2,12 +2,16 @@
 
 module CodeOcean
   class Config
-    def initialize(filename)
-      @filename = filename
+    attr_reader :path, :read
+
+    def initialize(filename, options = {})
+      @path = Rails.root.join('config', "#{filename}.yml#{options[:erb] ? '.erb' : ''}")
+      @read = parse(options)
     end
 
-    def read(options = {})
-      path = Rails.root.join('config', "#{@filename}.yml#{options[:erb] ? '.erb' : ''}")
+    private
+
+    def parse(options)
       if ::File.exist?(path)
         yaml_content = ::File.new(path, 'r').read || ''
         yaml_content = ERB.new(yaml_content).result if options[:erb]