I know it is easy to add "target=_blank" attribute in link tag, but I have to choose a most common way at exporting HTML, "open link in new tab" is not good default option base on some discussion in wordpress forum. Possibly, I may add an option in "Integration" preferences, to allow user enable this.
Moreover, CommonMark does not have this syntax although it is easy to add some new syntax, e.g, http://stackoverflow.com/questions/4425198/markdown-target-blank#answer-4705645. However I am very hesitate to add any new syntax over commonmark, just in case incompatible issue in future. Another similar case is, align the image to centre, it looks only add-on syntax can achieve that.