lazy load `Pry::InputCompleter` or `Pry::BondCompleter`

prior to commit both would be loaded but in most cases you'd only use
one or the other. when `bond` is available and `_pry_.config.completer`
has not been set to nil before the pry session has started & editline is
not being used(OSX only), then "pry/bond_completer" is loaded. otherwise
the default fallback is Pry::InputCompleter("pry/input_completer")