Now it's possible to have two lists of imports:
* extra_imports are imports that are always added to the context
* prelude_imports are imports that are added if we don't have
  any open modules in scope.

No UI changes or new commands are added for now.  This was functionality
that we needed in our customized GHCi at Facebook, so I wanted to get it
upstream to reduce the differences between our version and the upstream
version.