Skip copying paths that reference external assets

(Allows the use of google fonts with @import)

lib/showoff.rb

@@ -447,6 +447,7 @@ def self.do_static(what)
           File.open(css_path) do |file|
             data = file.read
             data.scan(/url\((.*)\)/).flatten.each do |path|
+              next if path.match('^http')
               logger.debug path
               dir = File.dirname(path)
               FileUtils.makedirs(File.join(file_dir, dir))