"Part Titles" in SUMMARY.md causes mdbook-graphviz to fail

[nickdanis]

mdbook supports markdown headings in the SUMMARY.md file as documented here: https://rust-lang.github.io/mdBook/format/summary.html

# Summary

- [Intro](./intro.md)
# Part 1
- [Chapter](./chapter.md)

When these are included in SUMMARY.md with mdbook-graphviz called in book.toml, the build fails with the following error:

Unable to parse the input
[WARN] (mdbook::preprocess::cmd): Error writing the RenderContext to the backend, The pipe is being closed. (os 
error 232)
[ERROR] (mdbook::utils): Error: The "graphviz" preprocessor exited unsuccessfully with exit code: 1 status

When either the part title is absent, or mdbook-graphviz is not called, the build succeeds as expected. This is using mdbook 0.4.10.

[dylanowen]

This was due to not supporting mdbook 0.4, you should be good now that I've published https://github.com/dylanowen/mdbook-graphviz/releases/tag/v0.1.0 with #4