Add ability to break by Word Count instead of target page count

As per a comment left on the plugin page http://www.oomphinc.com/plugins-modules/automatically-paginate-posts/, I've gone ahead and added the ability to specify a word count to break a page at rather than a total number of pages. This has no effect on existing setups, but a user can choose to break by word count instead.

Hope things are going well Erick!