Move `rmarkdown` to`Suggests` field

[dipterix]

I wonder if rmarkdown can be in Suggests field instead of Imports in DESCRIPTION, since has_pandoc is the only place using rmarkdown. You can dynamically check if this package has been installed via

# old
has_pandoc <- function() {
    return(rmarkdown::pandoc_available());
}

# suggest
has_pandoc <- function() {
	if(system.file(package = "rmarkdown") == "") { return(FALSE) }
    return(rmarkdown::pandoc_available());
}

I can create a PR if you like this idea.

[dfsp-spirit]

Yes, feel free to do that.

[dipterix]

@dfsp-spirit The PR #31 is ready. Also just to mention (in case), this edit complies with CRAN policy : )

[dfsp-spirit]

Thanks a lot, it's merged.

Just one minor thing: in this case it did not matter, but in general please do PRs against the develop branch, not master in the future.