Markyp-html

Mypy strict support by the addition of the py.typed marker.

Apparently py.typed was somehow missing from the previous version. It should be fixed in this one.

Added `py.typed` marker.

Fixed requirement parsing in `setup.py`.

Readme changes.

- Added `text.StyledTextFactory`.
- Added `__all__` to modules for better * imports.
- Added `block.embed`.
- Added a `placeholder()` utility method to `img` for generated placeholder images.
- Documentation updates and fixes.
- Minor bug fixes.