WKWebview 添加loading和estimatedProgress rx属性

WKWebview+RxSwift.swift

extension Reactive where Base: WKWebView {
    
    var loading: Observable<Bool> {
        return observeWeakly(Bool.self, "loading", options: [.initial, .new]).map { $0 ?? false }
    }
    
    var estimatedProgress: Observable<Double> {
        return observeWeakly(Double.self, "estimatedProgress").map { $0 ?? 0 }
    }
    
}