[Rule Request] Prohibited Interface Builder

[marcelofabri]

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.

Using Interface Builder (storyboards or nibs) is a constant subject of heated debates in the iOS community. Some projects don't use it at all. For those cases, a rule validating the use of @IBAction and @IBOutlet would be helpful.

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

// would trigger
@IBOutlet var label: UILabel!

@IBAction func buttonTapped(_ sender: UIButton) {}

// wouldn't trigger
var label: UILabel!

@objc func buttonTapped(_ sender: UIButton) {}

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

Just severity.

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.

Opt-in, as using IB or not is a project decision.