Allow custom non-breaking elements

[wallacio]

It would be great if it was possible to specify an element by tag/class/id, inside which content should not break (similar to issue #87 for anchor tags, really).

My primary goal is to prevent some other JS from breaking when the content is split, but I can see more generic uses of this.

If that's possible, it would also be good to be able to then specify whether the content should then break before, or after the non-breaking element.