Add configurability to match usage

[dagood]

As of writing, the prototype accepts a text/template file to fill in and write to disk, but all of the generated code is given to it as a single piece of data. It might make sense to improve on that, or maybe scrap the template for some other approach. With program arguments for things like package name, I think it wouldn't be hard to have the generator simply create the full file.

The way methods are filtered could also be improved. In the prototype, you pass a single regex. (| works.) My concern is that it could get unwieldy. A few ways forward:

• Use multiple go generate directives per distinct set of APIs.
  • Performance problem: the prototype scans some full tables to index them in memory, which would be repeated.
• Scan a Go file for //winmdsigs ... comments that tell the generator what to do. (Like //sys comments.)