Option `open` is incorrectly handled and/or documented

[Evgeny V]

According to the readme, theopen option is supposed to be a Boolean or an object of the { wait: false } format. However in the code it only checks for boolean.

https://github.com/forwardemail/preview-email/blob/4e48c73756b13599bbff92e30fe20a9bf1dd54e5/src/index.js#L55

If { wait: false } is passed it will always be true

[niftylettuce]

PR welcome!