Changed `--epub-toc-level` to `--toc-level`.

Also writerEpubTOCLevel -> writerTOCLevel.
So far this is only implemented in the EPUB writer.
Implemented --toc-level in LaTeX writer.
Implemented --toc-level for rst.
Changed --toc-level to --toc-depth.
Implemented toc-depth for html writer.
Epub CSS: left-align headers by default.

This is more consistent with the rest of the writers.