add sections automatically for gitbook SUMMARY.md from *.md source files

Add new sections by [GitbookIO editor](https://github.com/GitbookIO/editor) is quite simple, but sometimes you may forget to add sections in [SUMMARY.md](https://github.com/billryan/algorithm-exercise/blob/master/SUMMARY.md).

It is easy to parse the title of source .md files and add it to the appropriate chapter position. I will implement this feature with Python.

首先阅读 SUMMARY.md 和现有的源 .md 文件，以了解它们的章节结构和标题格式。然后确定 Python 工具将如何识别缺失的部分并将其放置到相应章节中；当新添加的源文件已在 SUMMARY.md 中体现且不影响现有顺序时，即表示完成。

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