Set header and footer subsection transitions independently.

[Lady-Glass]

I would like to individually configure config.footer.left.transition.name and config.footer.right.transition.name, so that I can use the left footer to show the name of items in a story and have that text fade in sync with the body text, while the right footer appears to remain static.

In the current version, setting the footer transition to crossfade causes the {restart link} to fade out and back in every time a new passage is printed. This is undesirable.