Configure anchor elements in configlet

Instead of defaulting to only h2,h3 as anchors, allow other items to be used
as well. This is done by allowing an administrator to supply a css3 selector
in the control panel configlet.
make anchor_selector GS configurable

This allows third party products to modify what tinymce considers to be
an anchor.
Merge pull request #44 from izak/master

Allow text other than headings to be used as anchors in the link plugin.