feat(@angular/cli): Support xliff2 for xi18n (angular#7734)

* Added support for the xliff2 serializer

Author: EvanCarroll

packages/@angular/cli/commands/xi18n.ts

@@ -17,7 +17,7 @@ const Xi18nCommand = Command.extend({
  name: 'i18n-format',
  type: String,
  default: 'xlf',
- aliases: ['f', {'xmb': 'xmb'}, {'xlf': 'xlf'}, {'xliff': 'xlf'}],
+ aliases: ['f', {'xmb': 'xmb'}, {'xlf': 'xlf'}, {'xliff': 'xlf'}, {'xliff2': 'xliff2'} ],
  description: 'Output format for the generated file.'
  },
  {