defer evaluation of yaml cell options

[jjallaire]

This PR makes my motivating example work correctly:

---
title: "test"
output: html_document
---

```{r}
x <- "Air"
```

```{r}
#| label: fig-test
#| fig-cap: !expr paste(x, "Quality")
plot(mtcars)
```

@yihui It's definitely possible that I haven't thought of everything required here as I don't have a full picture in my mind of the parsing/options/execution pipeline.

[yihui]

I think it should be good. Thanks!