Rule Request: Avoid optional Bools

[ejmartin504]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Rule Request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

This is somewhat similar to #1885 -- optional Bools are similar to optional collections in that an "empty" Bool rarely conveys much information.
I haven't found any existing discussions about this, but I have seen folks trying different solutions to dealing with optional Bools, all of which, to me, are not pretty. (see for example: Rule Request: Avoid optional collections)

2. Provide several examples of what would and wouldn't trigger violations.

Would trigger:

let foo: Bool?
let bar: Bool? = false
let baz = Optional.some(false)

These would not trigger:

// Properties should provide default value
let foo: Bool = false

// functions should return Bool instead of optional Bool
func foo() -> Bool

// input params should default to true or false instead of being optional
func foo(bar: Bool = false)

// Optional Bool's can arise from collections
let jsonData: [String: Any]
let foo: Bool? = jsonData["foo"] as? Bool

3. Should the rule be configurable, if so what parameters should be configurable?

No

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a
   rule as opt-in.

I'm curious what others think, but I would probably see this as an opt-in rule.

[marcelofabri]

Implemented in #2013