Modularize Rakefile into tasks, add sections, add update tasks

Add:
 * Brew update task
 * Vim update task
 * Vim's YouCompleteMe plugin install command
 * Additional information if an install task cannot run (ex: sudo, mac)
 * Section method which seperates the tasks and prints header info

Refactor:
 * Old way of using arguements for dry run and super user run, now we
   use ENV variables

Improve:
 * Rake tasks so it is possible to run a subset of the install/update
   tasks at a time
Broke brew install task into two: brew and brew_packages

Refactor:
 * Brew install task into two tasks, brew and brew_packages
Update Rakefile file comment, mention ENV vars