fix(task): fix that all editors can be started in config

I wasn't able to run a terminal-based editor process in my terminal (OS X 10.8.5, Terminal.app, Node v0.10.28), through the Grunt config `options.editor`, so I changed the way the editor process is forked (`child_process.spawn` instead of `child_process.exec`).

This works now pretty well and I tested the configuration with the following editors:

* `vim` (terminal based)
* `gvim` (window)
* `nano` (terminal based)
* `sublime -w` (window)

I couldn't figure out why `exec` doesn't work for my terminal based editors but I thought, it couldn't hurt to create a PR anyway, in case anyone else was affected.