New repl-env options: inject-html and inject-scripts

Default austin repl page did not support custom scripts and html. The patch adds two new options, so you can write:

```clojure
(austin-exec :inject-html "Hello, world!"
             :inject-scripts ["resources/js/my-favourite-lib/src/foobar.js"])
```

Thank you for writing Austin! I am happy that ClojureScript toolchain gets better and better every day.