make MAB not the default and change the config option for it

This is regarding our internal email where MAB is enabled by default, but that's not what most people will need or desire.

In any case, I've changed the `disable_whiplash` configuration to `enable_whiplash`, and modified the code for alternative choosing to be a bit more clear.

Perhaps this should be a test-level setting instead? Thats a separate issue I guess.

/cc'ing @jack7890 or my sixpack pull request will stay in limbo forever.