Generic Hiccup transformation fn

[simongray]

Most of the code in the hiccup->text function should be able to be re-purposed for a more generic transform-hiccup function which takes a similar input of conversions (matcher->zip-fn). Rather than returning string, the converted tree is returned.

This function should also be accompanied by some helper code for making XML Hiccup HTML-safe, i.e. element tags prefixed with a namespace and attributes prefixed with data-.

[simongray]

The hiccup->text could also possibly be turned into an implementation using this more generic function. We'll see.