Add ability to set transform output properties on the transform! function.

Each (propname, value) pair of strings in the output-properties argument
is set on the transformer (using .setOutputProperty) before the
transform is started. This allows access to special features of the
transformer, like configuring pretty printing or special modes for
(X)HTML.
Add core/alter-children for applying a function to child elements.

This returns the same element but with the child elements having the
value resulting from application of the provided function. The children
are passed into the function as a sequence, so wrap your function in
(partial map f) to apply it individually to all child elements.