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

[billryan]

Add new sections by GitbookIO editor is quite simple, but sometimes you may forget to add sections in 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.

[niangaotuantuan]

Also, when adding questions inside a section, it seems that manual update is needed, such as re-upload travis? I'm not sure about that...

[billryan]

Manual update for SUMMARY.md is necessary if you do not use Gitbook editor.

[niangaotuantuan]

get.

[billryan]

check_summary.py will update SUMMARY.md extracting from *.md title.