Move mutation processing into it's own class (#223)

* Move mutation processing into it's own object.

This should stand on it's own as a refactor, but is intended as a basis
for exposing the new MutationBuffer object to further outside control e.g.
to 'mute' or batch up mutation emission when the page becomes inactive
from a https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
point of view

* The `processMutations` function needed to be bound to the `mutationBuffer` object, as otherwise `this` referred to the `MutationObserver` object itself

* Neglected to add this output of `npm run typings`

* Get around the binding problem by using Arrow function expressions

* Prettier formatting

typings/record/mutation.d.ts

@@ -0,0 +1,20 @@
+import { mutationRecord, blockClass, mutationCallBack } from '../types';
+export default class MutationBuffer {
+    private texts;
+    private attributes;
+    private removes;
+    private adds;
+    private movedMap;
+    private addedSet;
+    private movedSet;
+    private droppedSet;
+    private emissionCallback;
+    private blockClass;
+    private inlineStylesheet;
+    private maskAllInputs;
+    constructor(cb: mutationCallBack, blockClass: blockClass, inlineStylesheet: boolean, maskAllInputs: boolean);
+    processMutations(mutations: mutationRecord[]): void;
+    private processMutation;
+    private genAdds;
+    emit(): void;
+}