Capture stdin

I'm trying to execute a couple node scripts via grunt that utilize [`prompt`](https://github.com/flatiron/prompt). However, grunt-execute just hangs, not recognizing the input.

I've attempted to fix by setting `{ stdio: 'inherit' }` in [spawn.opts](https://github.com/Bartvds/grunt-execute/blob/master/tasks/execute.js#L136) according to [child_proces](http://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options). This is close, but not exactly the same as the existing error handling.

Any other ideas?