Validate: Add subjectWhitelist option, excepting specific subjects

See the readme updats for usage details.

Fixes #26
Fixes #28 

@scottgonzalez @mislav would be great if you could take a look at this
I guess configurability is useful, but if you as a project maintainer decide to configure this value and forget to explicitly bring in the `fixup|squash` bit, you would be inadvertently breaking these git features again for other committers. Since there is really no reason why would you ever want to de-whitelist these prefixes, I suggest that they're hardcoded somewhere instead of being configurable. The square bracket syntax could still be configurable and turned off at will.