Feature/add blog slug

Adds a new variable available in the header: blog_slug

    {{ blog_slug }}

Converts blog_name into a category/tag friendly slug (all lower case, no spaces, just alphanumerics and dashes).

E.g.: "Tamouse's Extravagant Blog" => "tamouses-extravagant-blog"

As the blog name can contain just about anything, it can be problematic to use as a category / tag for jekyll.