Add app specific tag for posts/comments

[tibfox]

For statistics and dapp specific functionalities we should add a json metadata property to all posts/comments created via dtube go

[tibfox]

[brishtiteveja]

You think if you and my node merges with this dtube/avalon#61
possibly we can do multi tag ;)

[tibfox]

This feature was about adding a app metadata. Sorry the title was a bit confusing as all my issues I create as notes for myself ^^

Multitag feature is something I hoeatly would like to wait as long as possible on the app side ^^ stability first