Add <select>s for language and style of snippet

It would be nice if you could select language and style of the snippet. To be clear this would look like:


I would make a PR for this but I can't get the website to work locally. It seems like the README is missing some necessary steps, cause the /static/ files fail to load.

```
2021-11-04 01:44:22,072 WARNING django.server "GET /static/highlight.site.pack.js HTTP/1.1" 404 1825
2021-11-04 01:44:22,074 WARNING django.server "GET /static/demo/styles/mono-blue.css HTTP/1.1" 404 1834
2021-11-04 01:44:22,209 WARNING django.server "GET /static/demo/styles/mono-blue.css HTTP/1.1" 404 1834
```