Accept custom data.frame in write_refs

[LukasWallrich]

It would be great to accept a custom data.frame for write_refs ... or otherwise allow the user to change the tag naming.

It looks like the custom dataframe was supposed to be implemented (see below) - happy to submit a PR if you would like to allow this to be passed on to write_ris?

synthesisr/R/write_refs.R

Lines 146 to 149 in c406bc9

	}else if(inherits(tag_naming, "data.frame")){
	if(any(!(c("code", "field") %in% colnames(tag_naming)))){
	stop("if a data.frame is supplied to replace_tags, it must contain columns 'code' & 'field'.")
	}

[LukasWallrich]

I have now implemented this for use in CiteSource - happy to submit this function as a PR. Pls just let me know.