[Sass] Allow requires to be specified on the command line. This allow…

…s users to specify libraries that provide ruby-based sass functions.

lib/haml/exec.rb

@@ -317,6 +317,9 @@ def set_opts(opts)
         opts.on('-I', '--load-path PATH', 'Add a sass import path.') do |path|
           @options[:for_engine][:load_paths] << path
         end
+        opts.on('-r', '--require LIB', 'Require a ruby library before processing') do |lib|
+          require lib
+        end
         opts.on('--cache-location PATH', 'The path to put cached Sass files. Defaults to .sass-cache.') do |loc|
           @options[:for_engine][:cache_location] = loc
         end