Auditorium

* Improved support for running Markdown.

* Added support for running directly from Markdown files with `auditorium run <file.md>`.

* Added command `auditorium run <file.py>` for running a specific slideshow. This is now the preferred method.
* Added command `auditorium demo` for running the demo.

* Added support for `reveal.js` themes via `show.theme` and query-string argument `?theme=...`.
* Improved testing coverage.
* Fixed error with missing static files in the distribution package.

* Added support for fragments.
* Added support for vertical slides.
* Added custom layout options with `show.columns`.
* Added styled block with `show.block`.
* Added parameter `language` for `show.code`, defaulting to `"python"`.
* Improved layout for columns, with horizontal centering.
* Improved layout for input texts.
* Improved example for the `pyplot` support in the demo.
* Fixed some style issues.
* Fixed error reloading on a slide with an animation.
* Updated Readme with some examples.

* Refactor custom CSS and JavaScript into `auditorium.css` and `auditorium.js` respectively.