Rationale: we need this splitting level now not just in
EPUB but in chunked HTML.
`--epub-chapter-level` will still function as a deprecated
synonynm. `epub-chapter-level` will also continue to work in
defaults files, ande `epub_chapter_level` will still work for
Lua marshalling.
[API changes]
Text.Pandoc.App.Opt: remove `optEpubChapterLevel`, add
`optSplitLevel`.
Text.Pandoc.Options: remove `writerEpubChapterLevel`, add
`writerSplitLevel`.