html layer, clojure and company improvements, fixes and tweaks

• New contrib layer html (thanks to danielwuz)
• Improvements to the clojure layer: fancy symbols and align let. (thanks to nwolfe)
• Improvements to company-mode layer: support for include files (thanks to tux4life)
• New variable dotspacemacs-maximized-at-startup to maximize the window instead of making it fullscreen at startup. To make it fullscreen use the dotspacemacs-fullscreen-at-startup instead (thanks to bjarkevad)
• Fix support for fullscreen at startup for WIndows (thanks to ralesi)
• Add the global-mode-string variable to the powerline. This is a very important variable used by many modes (thanks to chrisbarrett)
• Add a section in documentation about the motivation behind the choice to use s instead of S for evil-surround (thanks to Tritlo for pointing out this major difference)
• Add missing link to source code pro in documentation (thanks to ralesi)