change pandoc_convert return.

[estedeahora]

Hi, I think it would be a nice improvement to pandoc_convert if, instead of returning NULL, it would invisibly return the command used in the pandoc call ('command').

This would only require modifying line 109 of pandoc.R to the following: invisible(command). I don't think this will create too many changes for current users and could be beneficial for use in other functions. On the other hand, I think it should be independent of 'verbose', as verbose prints to the screen, which is not a behaviour that is useful in all cases.