Add option to inject the JavaScript inside the HTML file

[joseprio]

I wanted to use this plugin to generate my JS13K entry; one common optimization is to inline the JS inside the HTML file... Is this something you would consider adding? If so, I'm happy to look into it and provide a PR.

[mentaljam]

I have added an ability to inject a text <script>. Is this enough for you or you need to inject a generated entry?