Add Another Plugin to *Convert* Linebreaks

We could possibly solve #1015058: Please Make WYSIWYG Editor Button Functional in a more elegant way by using a second plugin/button to convert linebreaks to markup, on the WYSIWYG attach action, but then leave them alone after the editor is attached.

Basically, do the first half of the JS functionality, and not the second. But we'd need to do some testing to make sure attaching/detaching multiple times doesn't screw things up, like it did before we fixed <hr /> and <table> tag handling...