Removes err highlighting for undefined code blocks

When the language for a code block is not defined and you are using Rouge for highlighting it will style everything with an `err` style and CGI escape the entire code block. This can be problematic for shell code blocks for example, `$ command --do --things 2> \dev\null` is rendered `$ command --do --things 2&gt; \dev\null`.

This pull request updates code filter to use the PlainText lexar by default if none is defined. It maintains the present settings if an unknown language is supplied.