renamed latex & HTML variables to be output-specific

Added:
- html-header-includes
- latex-after-packages-includes
- latex-header-includes

Renamed:
- `before-documentclass-includes` -> `latex-before-documentclass-includes`
- `before-packages-includes` -> `latex-before-packages-includes`
- `after-body-includes` -> `latex-after-body-includes`
- `after-document-includes` -> `latex-after-document-includes`