DangerDSL mocking [IDEA]

Hi, first of all thanks for this danger-js wrapper it is so cool 🎉.
I do have an idea that I'd like to hear your opinion about:

As I was developing a danger plugin I wanted to make it fully-testable. For doing that I had to mock DangerDSL which I spent a lot of time and the only way I found was initialising through JSON files. So if i have 200 tests I should have 200 JSONs (1000 lines each) . So i came up with this idea 👇

## Proposal
We can make use of Runtime library https://github.com/wickwirew/Runtime that will allow us to change let properties of dangerDSL😆.
At this point in my repo I have implemented this and working well.

```swift
/// Gives the ability to a struct/class to modify properties even let through runtime mirroring
protocol RuntimeModifiable {
/// Use this method to change a property on your RuntimeModifiable
/// ```swift
/// // In case you want to pass nil!
/// try mutablePR.rchange(property: "assignee", with: Optional.none)
/// ```
mutating func rchange(property: String, with value: Any) throws
}

extension RuntimeModifiable {
mutating func rchange(property: String, with value: Any) throws {
let info = try typeInfo(of: type(of: self))
let property = try info.property(named: property)
try property.set(value: value, on: &self)
}
}

// MARK: DangerDSL struct
extension GitHub.PullRequest: RuntimeModifiable {}
extension GitHub: RuntimeModifiable {}
extension DangerDSL: RuntimeModifiable {}
extension GitHub.User: RuntimeModifiable {}
...
```

and test now looks like this (in this test we make dangerDSL to have an assignee):

```swift
func testAssigneeExist() throws {
// Given
var fixture = githubWithFilesDSL()
var mutableGithub = fixture.github!
var mutablePR = fixture.github.pullRequest

try mutablePR.rchange(property: "assignees", with: [GitHub.User(id: 0, login: "", userType: .user)])
try mutableGithub.rchange(property: "pullRequest", with: mutablePR)
try fixture.rchange(property: "github", with: mutableGithub)

let sut = AssigneeExistsStep(danger: fixture)
// When
sut.execute()

// Then
XCTAssertTrue(sut.status.isSuccess)
}
```

By doing this we can stub what we want in dangerDSL in contrast of bombarding the whole package with JSON files.
Maybe this idea can be a seperate package (danger plugin) that can be used for easier DangerDSL mocking.

What do you think?