hook_purl() does not keep in cell options

[cderv]

As a follow up to

• knitr::purl does not retrieve comments that starts with #| in chunks #2268

fixed by 66d7700

Should we allow the same for hook_purl ?

Currently, it seems in cell options are lost.

[yihui]

This is a little tricky. In 66d7700, I stored cell options in the list element params.chunk of the parsed block, which can be accessed from tangle_block(). In hook_purl(), there is no way to access these options. I could store them somewhere (e.g., in knit_code), but I thought for a while, and felt it was a little messy to do. Unless there's a strong need, I tend to leave it behind...