Add option to the `explicit_self` rule to only enforce using self in initializers

Continued from https://github.com/realm/SwiftLint/issues/321.

I would like to see an option to only enforce self in initializers. I generally prefer using implicit self, but in initializers, it makes more sense to use self for consistency, as some properties might need self to differentiate them from the initializer arguments.

To illustrate, I would like to enforce:

class Unicorn {
	init(foo: String, goat: String) {
		self.foo = foo
		bar = goat
	}
}

To be:

class Unicorn {
	init(foo: String, goat: String) {
		self.foo = foo
		self.bar = goat
	}
}

Start with the existing explicit_self rule, its configuration, and its tests. Add coverage for an initializer-only option using the two examples in the issue, and consider the work done when the rule enforces self in initializers without changing behavior elsewhere.