Styling & functionality additions

Replace flexnav with slicknav, a seemingly better script for my purposes
Added category_gen.rb, category_index.html, archive_post.html, category_index.xml to allow generation of category archive pages similar to the existing site
Added submenu capabilities to the main menu and adjusted links.yml to include some sub menus
Added new .alignleft, .aligncentre, .alignright helper classes to main.css
Remove last of the initializr styles from h5bp.css
Changed site details in _config.yml
Updated header and body styling to better match what I am looking for in future