Adding support for multiple allowed exit codes for option.exitCode

This is a great an useful gruntplugin!

In my build process I have a 3rd party command that can both fail and succeed which is normal and a failure doesn't affect anything that should prevent the whole build from succeeding.
To support this I need to be able to provide an array of allowed exit codes for the command that grunt-exec executes, which is exactly what this commit does.

The feature does not break compatibility with existing versions because exit code can still be provided as before.

I hope you like it and will accept it :)