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.