[plugins.json] Add cocoapods-mangle

[James Treanor]

Please add the following entry to the plugins.json file:

{
  "gem": "cocoapods-mangle",
  "name": "CocoaPods Mangle",
  "author": "James Treanor",
  "url": "https://github.com/intercom/cocoapods-mangle",
  "description": "Mangling your dependencies symbols allows more than one copy of a dependency to exist without errors. This plugin mangles your dependencies to make this possible."
}

[Orta Therox]

Thanks, added with
7008b75

[James Treanor]

Thank you!