Support Flag for `copyToOverrrideDir`

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In some instances we may want to copy the fixtures from the base cache directory to the override directory. A flag would be useful, so as fixtures get used from the base, they are copied to the override directory.

new Mockingjays().start({
  copyToOverrideDir: (request) -> request.host.indexOf('auth') < 0 // only copy fixtures not part of the auth host.
});

new Mockingjays().start({
  copyToOverrideDir: true // Copy all fixtures to override directory.
});