Add danger-swiftlint (with Peril)

Adding this here and self-assigning so I don't forget: 

We have Swiftlint running successfully with no issues on CI, the problem is - it doesn't actually do anything with warnings. Only full failures will cause the CI to fail.

Since we already have Peril running cross-org, we can just add a Dangerfile with danger-swiftlint to comment everything needed on the PR itself. 