Mechanism to Override Templates

[Shinmera]

In order to make modules re-usable by administrators, templates and such need to be customisable without directly modifying the source. I'm not sure yet how this might be facilitated, but it's an important step to keep things properly encapsulated and easily extensible.

[Shinmera]

Using the change from #28, a new environment directory for template overrides could be used. The template-file function would then first check for the presence of an equivalent file in that directory, and fall back to the source directory otherwise. This would still require modules to be recompiled in case a template is added or changed, but that's a small price to pay. Will definitely need to be documented though as I imagine it would be confusing if the template overrides don't take place immediately.