Lightweight API for adding new `gather` commands

[andrewhead]

In Issue #16, @micahjsmith proposed a new type of gather functionality. nbgather could be made more extensible to let others build new plugins like that one, which do new things with the gathered cells.

I imagine the interface for adding would involve a registerGatherCommand function, where the caller provides a command title, an icon, and a callback that will get triggered and provided with the gathered cells.

Let's use this issue to track interest in having an API for registering new gather commands, and design discussions about what that API would look like.