Add support for ridge::linearRidge prediction

[Dan F]

I'm trying to get margins for ridge regression. To do that, I need the predictions package to support ridge::linearRidge. I have a PR I'll send for review.

[Dan F]

Here's the pull request: #45

[Thomas J. Leeper]

thanks!

[Dan F]

You're welcome.

However, I concluded I won't be able to make progress on margins for ridge. See leeper/margins#140 (comment).

I hope this change is still helpful in some way.