Wired Output format for R Markdown

Goal : create an output format for R markdown documents.

Exemple : https://github.com/dreamRs/wired/blob/435b6f78315c244da6d614f202d603d382833527/inst/rmarkdown/example.Rmd#L12

What we can do ?

* wrap plots in `wired-image`
* wrap other outputs in `wired-card` ?
* automatically add `wired_divider(2)` after a title `##` ?

What we have ?

A draft for `wired_document` based on `html_document` https://github.com/dreamRs/wired/blob/435b6f78315c244da6d614f202d603d382833527/R/markdown.R#L13

A custom hook for plots by [@cderv](https://github.com/cderv) : https://github.com/dreamRs/wired/blob/435b6f78315c244da6d614f202d603d382833527/R/markdown.R#L31

That need to be declared via `output_format` in `wired_document`, so users don't have to do

```r
knitr::knit_hooks$set(plot = hook_plot_wired)
```
After the hook can be used in chunk like that (must be declared by user this time) :

``````
```{r pressure, echo=FALSE, wired=TRUE}
plot(pressure)
```
``````

Could be nice to be able to set `elevation` parameter in `?wired_image` too