Ensure flexdashboard respects knitr's collapse option

[gadenbuie]

Fixes #102

Previously, flexdashboard was co-opting knitr's internal chunk hook. We now instead first call knitr's default chunk hook before applying flexdashboard's customized chunk hook.

Testing

The document below includes two R chunks that are either collapsed or not-collapsed and two figures (to demonstrate we haven't affected the figure syntax).

Before

After

Diff

Comparing the pre-rendered HTML output before and after the change (on the chunk with collapse = TRUE) shows that we've only affected the output. (FYI the diff is from a version with echo = FALSE.)

   <div class="knitr-options" data-fig-width="576" data-fig-height="460">
   
   </div>
-  <pre><code>[1] TRUE</code></pre>
-  <pre><code>[1] &quot;a&quot;</code></pre>
-  <pre><code>[1] &quot;b&quot;</code></pre>
+  <pre><code>[1] TRUE
+  [1] &quot;a&quot;
+  [1] &quot;b&quot;</code></pre>

knitr-collapse.Rmd

---
title: 'Messages collapsed'
output:
  flexdashboard::flex_dashboard:
    orientation: row
---

## Row

### Collapsed

```{r echo = TRUE, collapse = TRUE, message = TRUE, comment = "#>"}
knitr::opts_current$get("collapse")
print("a")
print("b")
```

### Not collapsed

```{r echo = TRUE}
print("c")
print("d")
```

## Row

### A wide plot

```{r, fig.width=10, fig.height=5}
hist(faithful$waiting)
```

### A tall plot

```{r, fig.width=5, fig.height=10}
hist(faithful$waiting)
```