Suggestion: drop the smart parameter from output formats

[atusy]

The output formats have the smart parameter (e.g., html_document).
This parameter is passed to smart_extension and adds +smart when TRUE and adds nothing when FALSE.

rmarkdown/R/output_format.R

Lines 349 to 351 in 43def55

	smart_extension <- function(smart, extension) {
	c(extension, if (smart && pandoc2.0()) "+smart")
	}

However, the smart extension is enabled by default in the Pandoc's markdown.

https://github.com/jgm/pandoc/blob/9feefa39319358acc84811e6017aeea60f9140c3/MANUAL.txt#L2627-L2628

This means, the smart extension is enabled regardless of the smart parameter (i.e., html_document(smart = TRUE) and html_document(smart = FALSE) cause the same result).

Thus, I suggest to drop smart parameter from the output formats.
By doing this, the user-specified smart parameter will be silently ignored as ellipsis.
If users really have to toggle the smart extension, they can do so via the md_extension parameter (i.e., md_extension = "-smart" disables the smart extension).
Currently, users have to specify both smart=FALSE and md_extension="-smart" to disable the smart extension.

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[yihui]

This can be a breaking change, but the fewer arguments we have to maintain, the better. So I just removed it. I think I'll need to tweak some functions to add the ... argument as you suggested after I run revdep checks of rmarkdown. Thanks!

BTW, this may also affect Pandoc 1.x users. I can't say for sure, but Pandoc 1.x probably didn't enable the smart option by default (and one needs to specify --smart in command line; cf #1168). Since Pandoc 2.0 has been released for more than two years, I think it's worth dropping the support for this tiny feature.

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