allow specifying which files to run with rake

Rakefile

@@ -2,7 +2,7 @@ require 'rake/clean'
 require 'pathname'
 
 task :spec do
-  spec_files = FileList.new('./spec/**/*_spec.rb')
+  spec_files = FileList.new(ENV['files'] || './spec/**/*_spec.rb')
   switch_spec_files = spec_files.map { |x| "-r#{x}" }.join(' ')
   sh "ruby -I./lib -r ./spec/spec_helper #{switch_spec_files} -e Minitest::Unit.autorun"
 end