Node validation

[mike-northsail]

Are there any plans to support node validation in the future?

• required inputs
• inputs meeting certain requirements (regex, url, email etc.)
• whitelist of output/input associations to limit node interactions by types

We're able to get around these by doing checks in a node's save/calculate methods while wrapping the InputOption to handle inline validation. Using the checkConnection event lets us validate against a map of allowed associations.

Great lib, thanks for putting this together and releasing it.