Added container class

The .container class acts like a wrapper, a bit like the .grid class, but without the flexbox properties.
You can now also add a class to your grid to make it full width: .grid.full-width.
I got the idea for this thanks to @jackmcpickle even though it wasn't exactly what he meant in issue #9, I think it might come in handy for some websites.

That's all!
Lucas