Add new template tag render_plugin_block

This templatetag acts like the templatetag 'render_model_block' but with a
plugin instead of a model as its target. This is used to link from a block of
markup to a plugin's changeform in edit/preview mode.

This is useful for user interfaces that have some plugins hidden from display
in edit/preview mode, but the CMS author needs to expose a way to edit them.
It is also useful for just making duplicate or alternate means of triggering
the change form for a plugin.

This would typically be used inside a parent-plugin’s render template. In this
example code below, there is a parent container plugin which renders a list of
child plugins inside a NAV block, then the actual plugin contents inside a
DIV.contentgroup-items block. In this example, the nav block is always shown,
but the items are only shown once the corresponding navigation element is
clicked. Adding this render_plugin_block makes it significantly more intuitive
to edit a child plugins content, by double-clicking its nav item in edit mode.
"explicit is better than implicit"

I'm not sure others would understand what was being returned without this.