Add Linter for CSS (fix #85)

[reficul31]

Refs. Added a CSS linter for the repository using stylelint. Currently it runs with the same command as the eslint but we can change it. There were only minor changes needed to be made to the css of the project as it was pretty much in line with the standard config provided by stylelint. For more info on the standard-config-stylelint.
There is one persisting error that I cannot resolve as I think it should be resolved by the maintainer. Please tell me any more rules are needed to be added.
NOTE Also there is no autofix in this particular linter.

edit: Sorry I couldn't integrate this with the eslint PR and had to sent a separate one. I was planning to do so but I took a lot of time it seems.

[Treora]

Nice, thanks! I rebased it to master, fixed (or rather suppressed) the remaining error, and added a few suggested rules. See branch css-lint. Feel free to reset to that branch and improve on it if you think there are other things to add, or say if you think it is ready to merge (I could make a PR to your branch, but PRs, and merging in general, don't work great with rebased branches).

Sorry I couldn't integrate this with the eslint PR and had to sent a separate one.

I much rather have separate PRs and merge them one by one, it makes reviewing and discussing so much easier.

[reficul31]

I had a look at the code. I think it works well with the CSS. I don't have anything more to add to this particular PR. Please feel free to merge. Thanks again for all the help!