Allow for column alignment for org table export

[mclements]

For table export to Org Mode, (i) the column alignment is currently lost and (ii) the output table includes colons which are not part of the Org Mode table specification. To address these issues, I have added some code to keep the column alignment and cleaned up the output table format.

I hope that this is helpful.

Sincerely, Mark.

[CLAassistant]

All committers have signed the CLA.

[yihui]

Thanks! Can this be achieved by the align.fun argument of kable_mark()? Manipulating the strings via regex feels like an ad hoc hack. Perhaps a custom align.fun function will be much more straightforward.

[mclements]

I have now updated this to use kable_mark(), with one call to gsub().

What is the expected behaviour when there are no column names? Specifically, should we include a top line? The current implementation removes both the blank column names (index=1) and the line (index=3):

if (is_null_colnames) res = res[-c(1,3)]