add tests to see if plugin loading fails appropriately

fiddle so that we can:
  optionally only load certain plugins
  optionally load all plugins except foo bar and baz
  default to loading all
the best way to do this is to have our own import() and do it at
use() time, so dumping Exporter

be paranoid about plugins trampling on each other?

what about plugins overriding default behaviour?
  hehe, Data::Compare::Approximate :-)
