Add plugins/generators via DLL

[vinniefalco]

It should be possible for MrDox to load generators at runtime via DLL "plugins", so that authors of new formats do not have to build LLVM in order to write and test them.

There's no rush for this.

[vinniefalco]

We have to attempt to make the metadata API such that generators do not need to include llvm headers, this will make it possible to write generators without installing and building clang/llvm.