Several improvements to the code generation functionality.
* Added PrecompiledHeader field to the configuration.
* Temporarily fixed an issue in HeaderConverter with the way DXGI.h apparently nests all of its types in an UnexposedDecl node.  This needs futher investigation.
* Temporarily made HeaderConverter emit a warning for unrecognized parameter annotations.
* Made the main program loop take into account the precompiled header.
* Added annotation support for optional in and out parameters and in-out parameters to HeaderConverter.