Formalize the script into a proper app

Formalize the script into a proper app to streamline maintenance
and installs.

This refactor includes the following changes and improvements:
* structures the app for gem deployment and adds Bundler utilities
* moves the main code into the GithubMarkdownPreview::HtmlPreview
class to allow other programs to include a preview
* adds comprehensive tests
* improves the scripts output (especially in error cases)
* updates to latest github css