Ability to customise base HTML `index.html`

**Is your feature request related to a problem? Please describe.**
I want to use an HTML snippet at the top of each of the output pages. In my cases, it's just some ASCII art (not super important) but I could see people wanting to place more useful things there. Currently I cannot add that because iles hides the underlying HTML.

**Describe the solution you'd like**
Since iles abstracts the base HTML, I cannot do so. Would it be feasible to replace the base HTML with a custom one that uses a slot where iles can inject its output code?

**Describe alternatives you've considered**
I'm looking for ways to use Vue to inject some text into the HTML. But I might just abandon it and not add the snippet at all.