Change the behavior of before_render and after_render hooks from #761.

This changes the before_render and after_render hooks so that the callbacks
given to them each get to modify the template source / the rendered output
in turn, instead of having to modify them in-place. It also changes the arguments
provided to the callbacks - now, they are given the source/output, the template path,
the locals hash, and the Tilt template *class*, wheras previously they were given the source/output
and the Tilt template *instance*. The locals hash is there in case pron case hooks want to
pay attention to the locals in some way. The before_render hook is also run before
compiling the template and caching it, which should prevent weird data mismatches
and save some template compilation effort.