[FR] Support split_by for section level higher than 2 (section) in gitbook

[lcougnaud]

Thanks for the nice package!

Currently a report created by the bookdown::gitbook function can be split at the chapter and section (or rmd) levels via the split_by parameter. Would it be possible to support the splitting of the report for section level > 2 (section)?

A proposal is available in this Pull Request.

I use this package to create a report with a high number of visualizations in different subsections, and the fact that the sections are rendered into separated html files (via the split_by parameter) efficiently reduces the report loading time.
With the extension to split into level > 2, the sections 11 and 12 in this example report could be grouped into a chapter and the graphs still exported into separated files by subsection (with split_by = '3+number').

Please let me know if this feature could be of interest for the package.

Thanks for the feedback!