Attach Nightwatch assertion messages to cucumber json report

[alcoforado]

I'm submitting a ... (check one with "x")

[ ] bug report
[x ] feature request
[ ] support request => Please do not submit support request here, instead use [Stack Overflow](https://stackoverflow.com/questions/tagged/nightwatch.js+cucumber)

Current behavior
Nithgtwatch assertions messages are not appended as attachments to cucumber json report

Expected/desired behavior
Append Nightwatch assertion messages as step attachments to cucumber json report.

[DimForCity]

+1

[jagadeeshshetty]

yes it's needed.

[mucsi96]

Can someone investigate this? A pull request is welcomed.