itemize: Don't break runs for font_features

We don't need to break runs just because font_features changed - we pass those to harfbuzz with the range they apply to, after all.

To do this, split the attributes into two lists, those that break runs (as before), and those that don't. And reapply the non-breaking ones after itemization.

For now, only font_features is considered non-breaking, but this can be expanded.

Closes: #242 (closed)