chore(TypeScript): refactor objects into classes

fix: `Nil`(null, undefined) now renders as empty string
change: `parser.parseValue()` renamed to `parser.parseOutput`
change: registered tags/filters become static and shared across different liquid instances

test/e2e/eval-value.ts

@@ -0,0 +1,14 @@
+var chai = require('chai')
+var Liquid = require('../..')
+var expect = chai.expect
+
+chai.use(require('chai-as-promised'))
+
+describe('.evalValue()', function () {
+  var engine
+  beforeEach(() => engine = new Liquid())
+
+  it('should throw when scope undefined', function () {
+    expect(() => engine.evalValue('{{"foo"}}')).to.throw(/scope undefined/)
+  })
+})