Generate single-file HTML from multi-file RA

[joanise]

Given the default multi-file output from readalongs align, possibly with manual corrections to the alignments in the SMIL or other manual changes, we want to be able to create the single-file HTML bundle.

Use case: -o html does not allow the user to do any corrections to their readalong. If they need to fix something but still want the results as a single-file HTML, we need to provide a tools to create the bundle post-hoc.