Allow user to specify build targets

I wanted to be able to control the platforms for which builds were created when running `grunt build`.  The current behaviour is to build for all targets (win, mac, linux32, linux64), but this is both time-consuming and I didn't find it very useful since I only use the one build for the platform I'm on.

Instead, let's seed the `nodewebkit` task with data we've already generated from `parseBuildPlatforms()` so that we only build the specified platforms.  This also means that now, by default, running `grunt build` will only build for the current system (i.e. if you're on Windows and you run `grunt build`, you'll only end up with a Windows build and no others).

In order to build for all platforms, you need to explicitly specify `grunt build --platforms=all`.

I'm still not *quite* happy with the formatting of the build instructions, so if anyone has any suggestions I'd be interested in hearing them.