Add comments to the "ghost backup" command

Hi guys,

First of all, thank you so much for making Ghost and adding comments + the native search. That is really awesome!

One thing that I noticed recently while moving my website is that comments are not included on the backup created by “ghost backup”.

Would be possible to include it in the future? I think I can save the backups by doing a full darabase backup, but I still think that would be useful to save them using Ghost CLI (ghost backup).

Thank you!