Support custom templates

[MelGrubb]

Make use of an assembly-level attribute to override higher-level items like the base class template, or to override templates on a global basis.

Consider allowing attribute parameters to override individual templates so that you could have multiple, alternative versions of certain templates, some of which get used for some classes, and some of which get used for others. I'm not sure this is as immediately useful, though. Interesting, but probably not worthwhile.