Closes#1905.
Removed stateChapters from ParserState.
Now we parse chapters as level 0 headers, and parts as level -1 headers.
After parsing, we check for the lowest header level, and if it's
less than 1 we bump everything up so that 1 is the lowest header level.
So `\part` will always produce a header; no command-line options
are needed.
