#4 Add += and -= operators on Counter API

[kkostov]

As proposed in #4, Extends the Counter API to enable increment and decrement of the counter's value using addition assignment and subtraction assignment operators e.g.

struct Schema: Codable, Equatable {
    var counter: Counter?
}

var doc1 = Document(Schema())
let _ = doc1.change {
    $0.counter?.set(3)
}

let _ = doc1.change {
    $0.counter -= 1
    XCTAssertEqual($0.counter?.get(), 2)
}

let _ = doc1.change {
    $0.counter += 1
    XCTAssertEqual($0.counter?.get(), 3)
}

[kkostov]

Awesome! I don't actually have permission to merge it by myself 😇