add as.iframe option in htmlwidgets

as.iframe_test.Rmd

---
title: "leaflet test"
author: "Hiroaki Yutani"
date: "`r Sys.Date()`"
output:
  html_document:
    as.iframe: true
---

source code of this Rmd is here: https://gist.github.com/yutannihilation/7d05bf4c34970258e693

## Install

```{r}
devtools::install_github("yutannihilation/htmlwidgets", ref="knit_print-with-iframe")
```

## No options

```{r echo=FALSE}
library(leaflet)

leaflet() %>% addTiles()
```

## ``widget.height = 300, widget.width = "50%"`

```{r echo=FALSE, widget.height = 300, widget.width = "50%"}
library(leaflet)

leaflet() %>% addTiles()
```

## ``widget.styles=list(`border`='10px solid red')``

```{r echo=FALSE, widget.styles=list(`border`='10px solid red')}
library(leaflet)

leaflet() %>% addTiles()
```