Speed up for parse class name (overtrue#150)

Speed up for parse class name

Author: kmvan

README.md

@@ -49,9 +49,9 @@ use Overtrue\Pinyin\Pinyin;
 // 小内存型
 $pinyin = new Pinyin(); // 默认
 // 内存型
-// $pinyin = new Pinyin('Overtrue\Pinyin\MemoryFileDictLoader');
+// $pinyin = new Pinyin('\\Overtrue\\Pinyin\\MemoryFileDictLoader');
 // I/O型
-// $pinyin = new Pinyin('Overtrue\Pinyin\GeneratorFileDictLoader');
+// $pinyin = new Pinyin('\\Overtrue\\Pinyin\\GeneratorFileDictLoader');
 
 $pinyin->convert('带着希望去旅行，比到达终点更美好');
 // ["dai", "zhe", "xi", "wang", "qu", "lyu", "xing", "bi", "dao", "da", "zhong", "dian", "geng", "mei", "hao"]