Work out how arbitrator contracts will advertise which templates they support

[edmundedgar]

Some arbitrator contracts, specifically the Augur and Kleros connectors, will want to limit which templates they support, as they need to be able to parse out the relevant parts of the question to them and send them to or check them in other systems.

We have a metadata field for things like this, but we need to define the form it should take, and add a check to the dapp to make sure you don't ask a question for an unsupported template.

While we're thinking about the metadata we should also do #151