Fixes #1 broken tests

Changes the test so it doesn't use the generation of h1...h6 tags, but just a plain paragraph.

That way, we avoid to test the generation of tag ids that became a default on a newer version of [marked](https://github.com/chjj/marked) for h1...h6 tags.
feat(#2): allows use of markdown external files (.md)

The README.md was updated with instructions on how to use the plugin
with external files or with markdown embedded in the html file.

Some demos were also created to depict the new usage possibilities.

Compatibility with the original format should remain the same - that
is, using markdown to render the **content of the slides**.
feat: adds gulp task for deploying demo

> gulp demo
To bring the demo up locally

> gulp deploy
To push the demo to github pages (username.github.io/bespoke-markdown)