Start preliminary work on RDoc 4 support

I wanted to use the Markdown parser in RDoc 4, but don't like the default templates. So I updated SDoc to work with RDoc 4. These are the bare minimum required changes to make the generator pass without errors. Some things don't work (correctly) yet:

- [ ] Github link now showing up
- [ ] Code blocks not working correctly (using Markdown)
- [ ] Probably more errors

Also, this was done pretty quickly, so it doesn't take into account backward compatibility. Keep that in mind.

If anyone wants to improve upon this, feel free to send pull requests to my branch.