Allow overriding default knitr chunk options

[gaborcsardi]

I think this would be useful to avoid repetition. I can try to write some test case.

Potentially fixes #1240

[hadley]

What about reading from knitr::opts_chunk$get() so we could avoid having to maintain the state ourselves?

What do you think the scope should be for the options? A single .Rd or the whole package?

[gaborcsardi]

IDK, it just seems simpler to specify the defaults in the config, and then use them in the whole package.

So I think the scope should be the whole package, for simplicity.

[hadley]

Yeah, that makes sense. Can you also add a few words to the rd-formatting.Rmd vignette in the code chunks section?

[gaborcsardi]

@hadley all done, I believe.

We need to run the test in a subprocess, unfortunately, because it messed up the roxygen pptions we rely on in other test cases. Alternatively we could add a knitr_chunk_options option to roxygen2 itself, maybe that's better?

[hadley]

Maybe we should have a local_roxy_meta_set()? Then I think you could just roc_proc_text() instead of having to set up a separate package?