Extended mode

- Implement prefix caching of extended mode
- Improved ranking algorithm for extended mode
- Fix nfc conversion bug

Rakefile

@@ -1 +1,8 @@
 require "bundler/gem_tasks"
+require 'rake/testtask'
+
+Rake::TestTask.new(:test) do |test|
+  test.pattern = 'test/**/test_*.rb'
+  test.verbose = true
+end
+