Fix implicit @section id, document @subsection and @backmatter

Allow omitting the id from `@section` to use a lowercased version of the name.  Prevents getting a `None` id causing a `NeglectedSections` error. Fixes #13.

Also adds missing documentation for `@subsection` and `@backmatter`. Fixes #55.
Doesn't it make more sense to replace the spaces with dashes?