code: def ensure_jekyll_dir
        @orig_dir = Dir.pwd
        start_path = Pathname.new(@orig_dir)
  
        ok = File.exists?('./_posts')
        new_path = nil
  
        # if it doesn't exist, check for a custom source dir in _config.yml
        if !ok
          check_custom_src_dir!
          ok = File.exists?('./_posts')
          new_path = Pathname.new(Dir.pwd)
        end
  
        if ok
          return (new_path ? new_path.relative_path_from(start_path) : '.')
        else
          puts 'ERROR: Cannot locate _posts directory. Double check to make sure'
          puts '       that you are in a jekyll directory.'
          exit
        end
      end
docs: Check for a _posts directory in current directory. If there's not one,
   check for a _config.yml and look for a custom src directory.  If we
   don't find one, puke an error message and die. If we do, return the name
   of the directory