[emacs] .dir-locals.el: add setup for js-mode

- move jshint option from gruntfile to .jshintrc
- remove trailing-whitespace from gruntfile and
- add jshint esversion: 6
- .dir-locals.el add locals for js-mode to use JSHint from the simple theme

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>