feat(subtitle): add a subtitle

contributed by @NickLee2050

re #11

Author: colinaaa

book.tex

@@ -19,7 +19,7 @@
 \renewcommand*{\lstlistingname}{代码}
 
 \title{何老师算法课笔记}
-\subtitle{还没有想好的副标题}
+\subtitle{Always be patient, sharp and diligent.}
 \author{计卓1801全体}
 \institute{华中科技大学}
 \date{\zhtoday}