Add back support for other syntax highlighting

We recently switched to use Rouge for syntax highlighting in the guides,
but in doing so we dropped support for highlighting the following
syntaxes, which are used in the guides:

apache
diff
json
markdown
nginx
scss
xml

There was also css and the "javascript" alias, but @eugeneius already
addressed these two.

They syntaxes were all being converted to "plaintext". This change allows
all the lexers mentioned above as well as any other lexers available
through Rouge.

We have deliberately decided to not support passing rougue custom lexer
options from the code fences. Instead any lexer options that are needed
can be set by adding an entry to the lexer_language case statement. This
should lead to more consistency in formatting and gives us more control
over what Rogue options are allowed.

guides/rails_guides/markdown/renderer.rb

@@ -68,14 +68,14 @@ module RailsGuides
 
         def lexer_language(code_type)
           case code_type
-          when "css", "js", "html", "ruby", "sql", "yaml"
+          when "html+erb"
-            code_type
-          when "erb", "html+erb"
             "erb"
           when "bash"
             "console"
-          else
+          when nil
             "plaintext"
+          else
+            ::Rouge::Lexer.find(language) ? code_type : "plaintext"
           end
         end