Added options to publish publishing pages. 

Approves if moderation is enabled. Unit tests also added. Updated the powershell commandlet with the same support. 
It might be smarter to specify the Publish parameter as a SwitchParameter instead of a bool. With a bool one would have to do Add-SPOPublishingPage -Publish:$true, with a SwitchParameter you only have to specify the -Publish parameter with no value.