[ci skip] Support switching mutant strategies easily
Exclude spec/spec_helper.rb from all rubocops
Support (re)defining chain specific failure chains

Such a failure chain will get invoked in case any
of the chain's handlers doesn't rescue from an
exception it raises.

Chain specific failure chains differ from processor
specific failure chains, in that they are meant to
be invoked for exceptional use cases only. They
make a chain "failsafe" by making sure that
Chain#call always returns a valid response, even if
the underlying code raised an exception.
[ci skip] Prepare changelog for release