Add -restrict option; restrict symbol generation to JSON file

Fixes go-gl/glow#42
I certainly did think about it. My thought was that it would end up being significantly more code and I wasn't sure if such a chance would be desirable. At any rate I don't think generation speed is particularly bad even with this method -- could we settle on a:
```
// TODO(slimsag): move post-generation filtering into pre-generation filtering
```
?