Add bundle.css support

[jenkin]

Hi, this cli is amazing for a frontend developer, thank you! :)

I added a feature very similar to bundle.js support, but for css. So the main template file can be simple and clean also in a large and complex presenter.

Here is my dev branch: https://github.com/jenkin/pbs/tree/feature/bundle-css

[teleyinex]

Hi, @jenkin,

NICE :-) However we're using webpack mostly, and by default, it populates our template with all our CSS, so that's why we didn't include it. Please, feel free to send a PR so we can merge it. Also, check our
Contrib guidelines to get your code in upstream.