Placeholder selectors post

Here is the finished post, intermediate felt like the right level to me. I've proof read it a couple of times but it might be worth a once over by someone else.

I couldn't get the site building as I've had issues getting ruby setup properly on Windows. I tried my best to match my syntax and format to that used in other parts of the repository. Some things that should be checked:

- I'm not sure if you have used `<pre><samp>` before on the site, it's used once in the article, ideally it shouldn't have syntax highlighting.
- I wasn't sure what the `slug` property was in the authors file, I just set `daniel-imms` but couldn't see it being used anywhere.
- Ensure the links are working, sublime text's syntax highlighting didn't want to color the last link 'mixing and matching' so I'm a little concerned about that.
- I set date published to the 9th of Feb.

Thanks! :)