Help text for node and section builders

[mxgrey]

While experimenting with #99 I found that it's difficult to know what to fill in for the config when setting up a node builder, or to know what the intended behavior of the node is.

Eventually we could probably use the config schema information to provide a more structured UI for filling in the config, but in the meantime it would be good if node and section builders could have a help_text field that gives a human-readable description of what the node's behavior is and how to fill in the config box.