Hide preview env values to prevent accidental deletion

[renkun-ken]

What problem did you solve?

Follows #1060 to address the same problem (#860) with previewing rmd.

(If you do not have screenshot) How can I check this pull request?

---
title: new document
author: test
---

This is a test document.

```{r}
rm(list = ls(all.names = TRUE))
x <- 1
y <- 2
```

Following is a test plot:

```{r}
plot(rnorm(100))
```
```

[ElianHugh]

Works nicely, thanks! LGTM