Adds RenderTo function, to render directly to a writer.

[drhodes]

Ok, I synced to your current master and merged. The pull request should now contain only what's relevant to RenderTo. The diff is much cleaner.

After pushing, I realized that the commit message is not what I wanted, tried git --amend, and pushed again, but the message didn't change the way I expected. Looking further, I found some options referring to git push --force as a fix, however it looks dangerous for pull requests.

If you'd like that message changed, I'll look into some more.