Toggle platforms to build with grunt cli option

Toggle platforms to build with grunt cli option instead of manual toggle in Gruntfile.js. This allows you to invoke grunt like this:

    $ grunt nodewkbuild

And it will build only for the current platform. Or like this:

    $ grunt nodewkbuild --platforms=win,linux,mac

And it will build for the specified platforms in the comma separated list.

This probably shouldn't be merged without fixing the regex for windows. It currently evaluates to true when process.platform is darwin (on os x). I can fix if you want this but just couldn't think of some nice solution for now...