Add debug and debug-brk executables

I wonder whether anyone would find this useful? As an alternative to setting `NODE_OPTS` to `--debug`/`--debug-brk`, or launching Grunt with ``node --debug `which grunt` ``. There is presently no easier way to enable debugging for Grunt tasks, e.g. for use with [`node-inspector`](https://github.com/node-inspector/node-inspector).