Just a few tweaks to @winner_joiner's answer above. I now have two NPP_EXEC scripts, each under a keyboard shortcut.
One creates an HTML version of the markdown file, with the same filename in the same directory, launches it in the default browser and shuts the console window. You then alt-tab over to the browser to admire the results.
SET OUTFILE = "$(CURRENT_DIRECTORY)\$(NAME_PART).html"
cmd /C \Python34\python.exe -m markdown2 "$(FULL_CURRENT_PATH)" > $(OUTFILE)
The other does the same, except does not launch it in the browser (
NPP_RUN $(OUTFILE) isn't there). I find this handy when I've already run the first script once: just alt-tab over to the browser and hit F5 to refresh it, rather than opening a new tab with the most recent edits.
You can save each script with it's own name, stick it in the Macro menu with NPP_EXEC's Advanced Options, and then assign a keyboard shortcut to it via Settings -> Shortcut Mapper.