Add an API for reading a single chunk from an external document

[multimeric]

This question has been asked many times on StackOverflow:

• https://stackoverflow.com/questions/24466121/include-a-specific-chunk-from-one-markdown-document-in-another-document
• https://stackoverflow.com/questions/52999022/run-selected-chunks-from-one-rmd-in-another
• https://stackoverflow.com/questions/43717908/is-it-possible-to-source-multiple-external-chunk-from-a-file-in-knitr-rmarkdown

In each case, the best or only solution is to use a hack like converting Rmd to R using knitr::purl, and then convert back to a chunk using knitr::read_chunk. This doesn't seem like an ideal solution, because firstly we're doing all this redundant conversion when we already have the chunk in the right format, it's just in the wrong place. Secondly, it requires the use of temporary files which it ideally would not, especially because we have to clean them up.

I wonder if we could simply have a function like knitr::read_rmd_chunk(file="foo.Rmd", chunk_name="bar-chunk") that would act like knitr::read_chunk in that it registers the chunk for use?

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[cderv]

I think this is a duplicated of

• https://github.com/yihui/knitr/issues/2041

Will track this in there if you're ok.

[cderv]

Duplicate of #2041

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