Remove magic for loading templates from module names

This specifies the template names in the exporter class, rather than getting them from the module names, which seemed a bit too magical.

Also, the latex templates got renamed from `latex/latex_foo.tplx` to `latex/foo.tplx`.

At present, when specifying a template to use, you have to specify the full template name, e.g. `html_basic`, not just `basic`. We probably want to think more about this, but I wanted to put the code up for review before I flew home, and I've had too many glasses of scotch to think about it just now. ;-)