Adding feature to whitelist domains and disallow as default behavior

[mimil]

Adding feature to whitelist domains and disallow as default behavior if no other rule has matched.

A new config option as been added:

• defaultType (String) Define the default behavior when a domain didn't match any rules, you can choose between valid or invalid (default is valid)

Here are tests added:

describe('Default validation should be configurable', () => {
    const ev = new EmailValidation()
    it('should say domain random.com is invalid because default validation is not set to accept, normally random.com should have been accepted', () => {
      ev.setOptions({ defaultType: 'invalid' })
      const result = ev.check('[email protected]')
      expect(result).to.be.an('object')
      expect(result.valid).eq(false)
      expect(result.errors[0]).eq('invalid')
    })
    it('should say domain random.com is valid because random.com has been whitelisted despite of default validation is not set to accept', () => {
      ev.setOptions({ defaultType: 'invalid', whitelist:['random.com'] })
      const result = ev.check('[email protected]')
      expect(result).to.be.an('object')
      expect(result.valid).eq(true)
    })
  })