Rethink how RSpec and Cucumber are run in different situations

The Guard configuration runs specs and features with debugger support
(specs only) and with progress-style output omitting exception
backtraces.

The 'test' Rake tasks run specs and features without debugger support
and with progress-style output, including a backtrace when exceptions
occur. The `rake test` command is invoked by `gem test` also in Travis
CI builds.

The 'spec' and 'features' Rake tasks run specs and features with
debugger support (specs only) and with documentation-style output
omitting exception backtraces.

The `spec` and `cucumber` commands run specs and features with debugger
support (specs only) and with documentation-style output omitting
exception backtraces.