Allow simple configuration of user-defined text parameters in node UI for extensions

[fvigo]

If a Minemeld extension requires the user to specify some parameter in the node (i.e. a username, a password, a URL), a specific webui extension must be written that defines how the entire UI for that node should look like.

While this is powerful and flexible, it's also complex when writing extensions that require simple user input of text parameters (strings) in nodes.

This FR is for a simple method to allow extensions to specify simple configuration parameters (i.e. text strings) that the UI will present automatically to the user in the node configuration, without requiring the extension developers to write lots of webui code.