Maybe support groups/brackets

[jonnor]

Required to construct (sub)streams, which is a concept in FBP for structuring data, using IPs that mark 'openBracket' and 'closeBracket'.
This is sometimes used in NoFlo components/graphs.

Right now one has to use custom test components in the graph fixture for these.

[bergie]

This is how we're commonly handling them in Mocha-land: https://github.com/noflo/noflo-flow/blob/master/spec/Gate.coffee#L34