title: What is extension_access_modifier swiftlint?
body: <p>I added Swiftlint to a project and I'm having trouble understanding what the warning is for <code>extension_access_modifier</code>.  I see it mainly on a class that is declared as public, but there are extensions littered throughout the codebase that adds functionality.</p>
  
  <pre><code>public class Foo {
  
  }
  
  // In SomeOtherClass.swift
  extension Foo { // Extension Access Modifier Violation: Prefer to use extension access modifiers
      public func baz()
  }
  </code></pre>
  
  <p>Whenever there is <code>extension Foo</code> in another class, I get that warning on the extension.  Would someone explain what it is?</p>