Support custom transforms for utterance

[dwhieb]

The user should be able to provide a custom transform for the utterance, which accepts all of the line data as one argument, and the utterance object as a second argument, and must return an HTML string.

This allows the user to add custom lines or other markup around existing lines, or to reorder lines. One specific use case is to add notes lines at the end of the interlinear example (which by default aren't processed).

This runs after individual line transforms are run.