Add comments about ngx_http_core_variables
Support passing nginx conf var between mruby code and nginx conf

- config

        location /check {
            set $fuga "100";
            mruby_set_code $hoge 'Nginx::Var.new.fuga.to_i * 2';
            mruby_content_handler_code '
              Nginx.rputs "fuga => #{Nginx::Var.new.fuga}"
              Nginx.rputs "fuga => #{Nginx::Request.new.var.fuga}"
              Nginx.rputs "hoge => #{Nginx::Var.new.hoge}"
              Nginx.rputs "hoge => #{Nginx::Request.new.var.hoge}"
            ';
        }

- result

$ curl http://127.0.0.1:8000/check
fuga => 100fuga => 100hoge => 200hoge => 200