SUMMARY.md support for non-list elements

[azerupi]

In SUMMARY.md support non-list entries:

For example:

# Summary

[Introduction](README.md)
[Foreword](foreword.md)

- [chapter 1](chapter_1.md)
- [chapter 2](chapter_2.md)

[Appendix](appendix.md)
--------------------
[Contributors](contributors.md)
[License](license.md)

• non-list link would be rendered without the section (chapter number)
• separator to separate some parts

[FuGangqiang]

what is the section number below?

[Foreword](foreword.md)
- [chapter 1](chapter_1.md)    # number == 1
[example](example.md)
- [chapter 2](chapter_2.md)    # number == 2 ?
[Appendix](appendix.md)

and

[Foreword](foreword.md)
    [chapter 1](chapter_1.md)   
        - [example](example.md)   # number == 1.1 ?
    - [example2](example2.md)     # number == 1 ?
- [chapter 2](chapter_2.md)       # number == 1 ?
[Appendix](appendix.md)

[azerupi]

I think non-list elements should be root elements only. But I am not sure if lists should reset or not. What do you think is better?

[FuGangqiang]

I think

• non-list should be root elements only
• not-list should not have subitem
• lists should not be break by non-list

that will not lead to ambiguity, and what do you think?

[azerupi]

Yes, that sounds good. :)