RMD Preview: font-size setting

[ElianHugh]

What problem did you solve?

Solves #1332 by injecting font-size in the preview HTML's css.

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

Render the following, modifying the r.rmarkdown.preview.fontSize setting or the markdown.preview.fontSize setting to change the size of text in the preview.

# H1

Foo.

## H2

Bar.

### H3

Baz.

#### H4

Foo.

##### H5

Bar.

```{r}
x <- 1
x
```

[renkun-ken]

Does it overwrite a themed document which has its own CSS font-size of those h1-h5 headers?

[ElianHugh]

It would depend on the html theme's css -- if it has higher priority styling then no. typically yes this will override that.
admittedly, i'm not sure how to handle that, should themeing be respected in previews?

[renkun-ken]

Not sure if we should hard-code some font-size adjustment or just use some zoom parameter?

[renkun-ken]

I tried with some scenarios, looks like adjusting font size may result in many size inconsistencies between different HTML elements such as normal text, headers, images, code blocks, etc.

I find using zoom seems good enough, as suggested in #1348. @ElianHugh What do you think?

[ElianHugh]

That sounds fair, it will work in more situations compared to just font size