SUMMARY.md can no longer contain HTML comments

[Michael-F-Bryan]

In one of my books I comment out not-yet-written chapters using HTML comments. After updating from v0.3.4 to v0.3.5 running mdbook build now fails while parsing SUMMARY.md.

$ mdbook build
2019-11-13 10:00:58 [ERROR] (mdbook::utils): Error: Summary parsing failed
2019-11-13 10:00:58 [ERROR] (mdbook::utils):    Caused By: There was an error parsing the numbered chapters
2019-11-13 10:00:58 [ERROR] (mdbook::utils):    Caused By: Unable to get last link because the list of SummaryItems doesn't contain any Links

I believe the regression was introduced in #1021 when pulldown-cmark was updated.

[Michael-F-Bryan]

Minimal, reproducible example:

# Summary

- [First](./first.md)
<!-- this is a comment -->
- [Second](./second.md)

[EasiestSoft]

I also hope this will be resolved soon