[FEATURE REQUEST] Source metadata about vertices.

[LdDl]

Is your feature request related to a problem? Please describe. Extend the way we store edges/vertices data.

Describe the solution you'd like and provide pseudocode examples if you can Exported CSV is human readable, but we need to make an extension for this to store source metada. Also we should add metadata fields for vertices/edges and (may be?) restrictions. Some getters/setters workaround is needed.

Describe alternatives you've considered and provide pseudocode examples if you can Nope

Additional context It's would be good to store metadata in JSON'ish string or other popular "key-value" format.

[LdDl]

https://github.com/LdDl/horizon/issues/1

Example of real-world application.