Decide on a convention for re-frame handlers registration

[julienfantin]

1. Should we provide a registration function that registers the pull handler?
   Yes: nobody likes setup
   No: we'd need to answer the remaining questions

2. If yes, should we define the sub id?
   Yes: it's good to standardize usage
   No: we'd need to answer the remaining question

3. If yes, should the sub id be fully-qualified?
   Yes: everything should be, and it forces a require which means we could register as a top-level
   No: Right now it'd be :vimsical.subgraph.re-frame/pull which I can't think of a good alias for