Add (somewhat messy) ErlyDTL support

I kinda took the easy way out and just had it add ErlyDTL modules to the source files list like everything else, and then detecting whether to use ErlyDTL or Erlang's compiler whenever it compiles them.

There's a bit of hackery to get the module names right, as Erlang doesn't seem to be a fan of source files that don't end with .erl (so ErlyDTL modules report their source to be `<filename>.dtl.erl`).

There's also an issue where changing a template doesn't recompile templates that extend the base template that changed.

Overall, this could be done better, but I need to get work done on my own project, so this will suffice for now. I might go back and do it properly at some point, but probably not any time soon.