Update devtools
Remove unused class
Simplify method expansion
Add Mutant::Runner::Subject

The idea is to break up the big fat and ugly Runner class into dedicated
pices with a single reponsibility.
Cleanup killers
Use composition library
Spec Runner::Subject#mutations in dm2 style
Add runner objects for config and mutation
Move CLI parsing specific stuff in subclass

Not ideal, but a first step to improve SRP here.
Use reflective codeclimate badge