Switch --gist output to gisthost.github.io with backward compatibility (#31)

> The --gist option should output a link to gisthost.github.io rather than gistpreview.github.io
>
> The JavaScript logic injected into the pages should now work for both of those domains, so it should detect if the page is being served on gisthost.github.io OR gistpreview.github.io

- Update preview URLs to use gisthost.github.io instead of gistpreview.github.io
- Update JavaScript to detect both gisthost.github.io and gistpreview.github.io domains for backward compatibility with existing links
- Update help text and README to reference the new domain

https://gisthost.github.io/?e33364139d39625eda6fbbe8527f5902/index.html