Radspec: allow users to define extensions at run-time

[sohkai]

Radspec descriptions are usually very organization-specific (contextual to each individual organization).

This would require aragon/radspec#88, but we should expose a way for Connect users to declare any additional radspec descriptions they'd like to incorporate into their organization's actions.

Another aspect I was thinking about was making the action descriptor transformation run-time configurable; an example might be an organization that is backed by a database (or IPFS blob) mapping descriptions to transaction hash or action id (e.g. voting app addr + vote id).