Adding better handling for following (+) selectors

Similar to the nth-of-type selector, the following (+) selector wasn't being honored for content that was split across pages (the first half of the paragraph would be styled correctly, but the second half would not). I went ahead and added pretty much the same handling as here: https://gitlab.pagedmedia.org/tools/pagedjs/merge_requests/122.

Specs are included, and the pipeline should pass. @fchasen ready for review!