Add sink method to quickly attach a MMMLoadableObserver

So it now passes the actual loadable subclass instead of a generic
MMMPureLoadableProtocol inside the closure.

MMMLoadable.podspec

@@ -6,7 +6,7 @@
 Pod::Spec.new do |s|
 
   s.name = "MMMLoadable"
-  s.version = "1.6.4"
+  s.version = "1.7.0"
   s.summary = "A simple model for async calculations"
   s.description = "#{s.summary}."
   s.homepage = "https://github.com/mediamonks/#{s.name}"

Sources/MMMLoadable/MMMLoadable.swift

@@ -13,3 +13,18 @@ extension MMMLoadableState: CustomDebugStringConvertible {
 
 	public var debugDescription: String { NSStringFromMMMLoadableState(self) }
 }
+
+extension MMMPureLoadableProtocol {
+
+    /// Observe changes in this loadable. Will stop listening to changes when
+    /// ``MMMLoadableObserver/remove()`` is called or the observer deallocates.
+    /// 
+    /// - Parameter block: Get's called every time the loadable changes.
+    /// - Returns: The observer, you usually want to store this outside of the scope, e.g.
+    ///            in a private property so it doesn't deallocate right away.
+    public func sink(_ block: @escaping (Self) -> Void) -> MMMLoadableObserver? {
+        return MMMLoadableObserver(loadable: self) { loadable in
+            block(loadable as! Self)
+        }
+    }
+}