Ability to edit node metadata

[jonnor]

This is useful for instance to specify number of workers for a particular node for instance, or the autoscaling configuration.
It may make sense to have a general editor first, and then possibly add task-dedicated UIs in addition/on-top after that.

Requested for ElexirFBP: http://stackoverflow.com/questions/30939057/adding-additional-metadata-to-the-noflo-ui
At The Grid we would like to use this in our backends MsgFlo and guv autoscaling.

[jonnor]

Proposed way to implement would be to add it to the node-inspector card, that appears on right-hand side. The code for that can be found at: https://github.com/noflo/noflo-ui/blob/master/elements/noflo-node-inspector.html
Possible UI could be to have a list of key+value pairs, with actions to add, edit and delete such a pair.

This would be resonably general, without going all the way to a JSON textfield editor...

[bergie]

Down the line we could provide a mechanism for runtimes to give us a metadata JSON schema, so we could provide a more specific editor (selects, typed inputs)