Don’t include HTML dependencies (from R execution) unless we’re targe…

[dragonstyle]

…ting HTML output

This should fix #1378

Testing with:

---
always_allow_html: true
---

```{r}
htmltools::tagList(rmarkdown::html_dependency_font_awesome())
htmltools::div(class="dummy", 
               htmltools::p("HTML content"))
```


```{=html}
<i class="fas fa-user"></i>
```

Which should properly output a font awesome icon for HTML while not including any HTML output (including in header) for pdf/latex.