Hack to center code which does not belong to any language

- From dom selecting all code elements with class `plaintext`and changing its parent to `p` . 
Note : other elements have language class instead of `plaintext` class
- Slate feature of empty language (showing in all language) wont work
- Adding bug fix for broken link
 
It looks like this doesn't preserve indentation. It's better to have named values instead of using `$2` `$3`. Here's [an example](https://github.com/gollum/gollum-lib/blob/bb43d34e4ee1278f270d73a765e9b879f32723e0/lib/gollum-lib/filter/code.rb#L10).