Document async option

[errnesto]

The executefunction takes a second parameter sync.

When this is used it calles spawnSync and then calls next() when it is finished.
If no option is provided it calls spawn and attaches an .on('close') handler that calls next().

If I don't missunderstand it, in both cases one command is run after the other succeeded.
Why would I want to use sync?

[audinue]

Please read #7. Thank you.