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

src/parser/parse-stream.ts

@@ -0,0 +1,47 @@
+import Token from 'src/parser/token'
+import ITemplate from 'src/template/itemplate'
+
+type parseToken = (token: Token, remainTokens: Array<Token>) => ITemplate
+type eventHandler = ((arg?: Token | ITemplate) => void)
+
+export default class ParseStream {
+  private tokens: Array<Token>
+  private handlers: {[key: string]: eventHandler} = {}
+  private stopRequested: boolean
+  private parseToken: parseToken
+
+  constructor (tokens: Array<Token>, parseToken: parseToken) {
+    this.tokens = tokens
+    this.parseToken = parseToken
+  }
+  on (name: string, cb: eventHandler) {
+    this.handlers[name] = cb
+    return this
+  }
+  trigger (event: string, arg?: Token | ITemplate) {
+    const h = this.handlers[event]
+    if (typeof h === 'function') {
+      h(arg)
+      return true
+    }
+  }
+  start () {
+    this.trigger('start')
+    let token
+    while (!this.stopRequested && (token = this.tokens.shift())) {
+      if (this.trigger('token', token)) continue
+      if (token.type === 'tag' &&
+          this.trigger(`tag:${token.name}`, token)) {
+        continue
+      }
+      const template = this.parseToken(token, this.tokens)
+      this.trigger('template', template)
+    }
+    if (!this.stopRequested) this.trigger('end')
+    return this
+  }
+  stop () {
+    this.stopRequested = true
+    return this
+  }
+}