fix: missing variable declaration (#660)

### Issue

<img width="491" alt="screenshot 2018-10-30 at 23 31 35" src="https://user-images.githubusercontent.com/8396515/47757020-58587080-dc9c-11e8-867c-dc3feabcb651.png">

Sorry, forgot to include this line in previous PR.

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Author: cheng-kang

src/core/render/compiler.js

@@ -71,6 +71,7 @@ export class Compiler {
     this.router = router
     this.cacheTree = {}
     this.toc = []
+    this.cacheTOC = {}
     this.linkTarget = config.externalLinkTarget || '_blank'
     this.contentBase = router.getBasePath()