feat: new option to turn gallery generation off

[mojoaxel]

This introduces a new option gallery which is true by default.
If gallery is set to false and server.start is also false then the outputFolder gets removed completely (clean) after the compare.

Also the callback now returns the highest mismatch percentage found. This is useful if you want to run wraith quiet inside automated tests. e.g.:

config.quiet = true;
config.gallery = false;
new Wraith(config, function(maxMissmatch) {
  assert.equal(maxMissmatch, 0);
});

All changes are compatible to existing configs. Therefore I recommend a minor version change.