feat: allow to modify raw preset config

[ktsn]

ref #71

We can modify a preset config object by specifying modifyConfig function in preset field.

module.exports = {
  preset: {
    name: 'foo-preset',
    modifyConfig: config => {
      // remove js config
      delete config.rules.js
    }
  }
}

Also I made rules field can accept null or undefined value for the users can remove preset rules by just assigning such values.