Consider emitting "MARK" comments between code sections

[JakeWharton]

Since the members of a class/struct/enum are emitted in a particular order, "MARK" comments could be produced between the sections to aid in navigation in xcode.

Something like

class Foo {

  // MARK Public Properties

  public var bar: Bar

  // MARK Private properties

  var baz: Baz

  // MARK Initializers

  ..

  // MARK Public functions

  ..

  // MARK Private functions

  ..
}