Lint for implicitly unwrapped optionals

Nobody has claimed this yet.

To prevent regressions of #228 like #447, we should install [SwiftLint](https://github.com/realm/SwiftLint/) and configure it to warn on `force_unwrapping` in the four framework targets, then install Danger to surface the warning. Other linting rules and the example and test bundle targets are out of scope.

I think warning on `force_unwrapping` is a reasonable step in this repository specifically, because this project primarily encodes and decodes data and requests and receives HTTP responses. This library is designed to give the developer as much control as possible, including control over error handling, so a force-unwrap is almost never appropriate.

/cc @mapbox/navigation-ios @dmiluski

Start by locating the project configuration for the four framework targets and the existing integration point where Danger can surface warnings. Configure SwiftLint to warn on force_unwrapping and connect that warning to Danger; done means the four framework targets are covered while other rules, examples, and test bundle targets remain out of scope.

Written by the indexing model from the issue text.