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

If anyone wants to improve upon this, feel free to send pull requests to my branch.