added experimental option to customize EntitySelector's source selection

the goal here is to let the user choose which remote sources to query in
a federated scenario

the code is little more than a primitive mock-up, using `confirm` to
simulate multiple choices - in the actual implementation this would be a
proper dialog (at the end of which `callback` would be invoked)

caveats:
* `sourceSelectors` seems like over-engineering - this might better be
  solved with a subclass
* the assumption is that no matter how many sources were selected, only
  a single HTTP request would be triggered - thus requiring the iQvoc
  proxy to aggregate and combine the data from various sources;
  triggering an individual request for each source would require more
  refactoring (`getJSON` -> aggregate -> autocomplete callback)
* making the user limit the sources seems like premature optimization;
  it'd be easier for everyone to just query all sources and present the
  results to the user (e.g. in a tabbed UI?)

/cc @mrreynolds