added "rmarkdown" engine to process code blocks

[dgkf]

I ran into a situation where I wanted to be able to turn on or off different pieces of commentary in the markdown throughout a document. For example, I would ideally like to set a flag to include more technical commentary for an analytic audience, but then strip them out for a general audience.

Although the asis engine works in most cases, it doesn't process inline code blocks that I often use for communicating results. To address this, I added a new rmarkdown engine to pre-process the text using knitr::knit.

Here is an example document where this is useful:

---
title: "test"
output: pdf_document
---

```{r, echo=FALSE}
# create a flag to easily turn on/off markdown 'code' blocks
.show_analysis_commentary <- TRUE
```

this is some general text

```{r analysis, echo = FALSE}
# derive answer to life, the universe and everything
var <- 42
```

```{rmarkdown, echo = .show_analysis_commentary}
# print out analysis result only if `.show_analysis_commentary` is `TRUE`
the answer to life, the universe and everything is `r sprintf('%.f', var)`
```

```{r}
hist(mtcars$mpg)
```

---

let me know if any tests should be added. I couldn't find any for the asis engine and figured this would only necessitate a similar level of testing.

[yihui]

Thanks! I don't have time to think about it carefully at the moment. You should probably call knit_child() instead of knit() here. Anyway, I guess the problem should not really block you, because there are at least two ways out:

1. You can register the engine by yourself via knitr::knit_engines$set(rmarkdown = function(options) ...);

2. Or use child documents (which will require the commentary to be written to external files, and you may not like that).

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