Markyp-bootstrap4

Fixed requirement parsing in `setup.py`.

Changes:
- Added the `navs` module.
- Added custom keyword arguments to `layout.row()`, `layout.one()`, `layout.container()`, `layout.container_fluid()`, and `layout.row_item()`.
- Added the `navbars` module.
- Added the `scrollspy` module.
- Added the `pagination` module.
- Some minor bug fixes.
- Added examples of all new features to `demo.py` and the demo page (on GithubPages).
- Added tests.

Added:
- `list_groups`
- `modals`
- `tabs`

- Updated `layout.autocol()`.
- `dropdowns.menu_item()` got a `factory` argument.

Added `SubmitButtonFactory` and `submit_button` to `forms`, it was badly missing.

- Added `__all__` to modules for better * imports.
- Added `cards`.
- Added `PercentSize`, `offset`, `row_break` to `layout`.
- Added `carousels`.
- Changed the argument list of `margin()` and `padding()`.
- Added `collapses`.
- Added `dropdowns`.
- Added `input_groups`.
- Added `forms`.
- Added `jumbotrons`.
- Added content to the Getting started section of the readme.
- Set up GitHub Pages for the project.
- Bug fixes and documentation fixes.