Support extending json formatter with text diffing

[atsjo]

This is NOT a request to add non standard text diffing operations to JSONFormatter, but to change the signature of the class so that users can add it themselves by extending it... Thanx for the recent fixes for moves and text diffing @benjamine, much appreciated!!!

I prepared a PR for this, like I implemented it myself: #414

I also prepared a PR for exporting parseTextDiff as function (in addition to a method on BaseFormatter), so its easier to import when extending JSONFormatter: #415

[atsjo]

I have to import parseTextDiff like this today

const parseTextDiff = BaseFormatter.prototype.parseTextDiff;

which includes the whole BaseFormatter in the bundle, even though only that function is used...