Build: Use jsHint extends option

jsHint 2.4.0 adds an option to extend another configuration file,
making it possible to extract common configuration to a separate file.

Note: I'm not happy with the `assign` hack so it might be better to wait until https://github.com/jshint/jshint/issues/1439 is fixed. Nonetheless, I wanted to get some feedback so I'm putting it here.

Fixing https://github.com/jshint/jshint/issues/1439 will let us not only not add the hack I had to add here but also remove the one with deleting the `onevar` option - it's better to just overwrite it in `dist/.jshintrc`. All our jsHint configuration can then be merged to one target instead of two we have now.