Allow custom HTML head

[georgefst]

This is a proof-of-concept breaking version. I'm not quite sure what a good API would look like.

I have found this useful, particularly for specifying stylesheets.

[georgefst]

An alternative, which I'm using for now, is to modify the head imperatively with jsaddle-dom. For example, these are essentially equivalent:

main = debugOr
    (Just $ encodeUtf8 $ TL.pack "<link rel='stylesheet' href='style.css'/>")
    port
    start
    app

main = debugOr
    Nothing -- passing `Nothing` here is the same as using the old version of `debugOr`
    port
    (insertStylesheet "style.css" >> start)
    app
  where
    insertStylesheet stylesheetUrl = do
        Just doc <- currentDocument
        Just headEl <- getHead doc
        linkEl <- unsafeCastTo HTMLLinkElement =<< createElement doc "link"
        setRel linkEl "stylesheet"
        setHref linkEl stylesheetUrl
        appendChild_ headEl linkEl