Allowing prefix and suffix in file generation

I may be wrong, but elm doesn't generate a complete HTML page (it doesn't have the `` for instance) so I usually do
```bash
elm-static-html --filename Static.elm --output static.html
cat header.html static.html footer.html > index.html
```
but this doesn't scale very well with multiple file generation. Should we allow for this library to automatically handle this kind of things?