Can now export lookatme slides with `--to-html OUTPUT_DIR`!!!

[d0c-s4vage]

Todos in future iterations:

• Make it look good on mobile, and add way to control slides on mobile
• Copy code to clipboard when clicked

Example

The result of running the command below is attached:

lookatme --tutorial --to-html /tmp/tutorial_html

tutorial_html.zip

Screenshot of loading tutorial_html/index.html in Google Chrome:

[d0c-s4vage]

This PR should address both #154, #155, and #196