Convert chunk option to YAML

[cderv]

This close #2082 and follows #2149

Chunk options are parsed and then written to YAML using yaml::as.yaml() with two tweaks to the parsed chunk options

• Logical will be true or false instead of default yes or no
• R expression or variables passed in chunk option value will be written to YAML after !expr to make the fact they should be evaluted before parsing. Example:

```{r, R.options = list(width = 80)}
names(knitr::knit_engines$get())
```

will be converted to

```{r}
#| R.options: !expr list(width = 80)
names(knitr::knit_engines$get())
```

Doing

R.options:
    width: 80

is a bit more complicated than the current solution, as it implies going through the all tree of expression to only find the leaf is list that are variable or expression to evaluated with !expr

This solution is simple enough to produce valid document, and it is easy enough for the user to reformat manually some chunk header.

Here is how a conversion looks like on all knitr-examples: yihui/knitr-examples@master...cderv:knitr-examples:yaml-chunk-header
Does it seems right to you ?

It seems there is still an indent question where the chunk header is not indented, whereas the code inside is - do we want to indent the in-chunk option part too.

Anyway, @yihui I let you review. I wanted to have something ready for the conference, so here it is.

[cderv]

Tests are failing because r-forge seems down... and some examples uses it.

[yihui]

Looks perfect. Thanks!

[yihui]

I think simply tolower(x) would work. Correct me if I'm wrong.

[cderv]

Oh yes you're right it should work woth the automatic logical to character conversion.