System.REPL.Asker now uses MonadError
Generalized Fetch.ErrorHandling to Showable errors.
Changed Either to MonadError in System.REPL.Commands.

All the functions in System.REPL.Commands now use MonadError-contexts
instead of returning values of type m (Either (AskFailure e) z). This
significantly simplies the logic.
Add Crawler configuration.
Added Data.List.Safe.!!
Deleted System.REPL.Command.Helper.
Deleted :gallery command, making it a crawler.

Relates to #36.