Add knit_value S3 method for overriding inline R chunks

I assume you would do this a little differently, and it's not documented, but I wanted to submit this so you could think about it. I couldn't get the `as.character` overriding approach to work as we had discussed it--I get some kind of vapply error when I try, and also the `knit_meta` would not be passed along properly (which is necessary for e.g. `sliderInput` to work). So instead I added `knit_value` which is analogous to `knit_print` but for the inline case.