Added check-bare to the Makefile for doing an unvalidated build

The reason I'm adding this option is because whilst adding features to the website, I often go through iterations of adding content and rebuild to check that the website looks okay. I find myself always stopping make when it gets to the validation step because early on I just don't care if it's valid or not (it probably is, and if not, I'll fix it up later once the site looks as it should).