refactor(strip_html): remove striptags deps

[SukkaW]

striptags is the most safe strip HTML lib (as it implements a AST parser instead of using regex).

However:

• striptags implements a simple Symbol polyfill that Hexo doesn't need (we target latest Node.js LTS)
• striptags supports stream and many other features (like tags whitelist) that Hexo doesn't need as well
• In order to supports those features striptags introduce a bit performance drawback.

The PR re-implements the striptags without those unneeded features. The unit test cases are adopted from striptags.

[coveralls]

Coverage increased (+0.2%) to 97.661% when pulling 732602d on SukkaW:strip-html into 19315b8 on hexojs:master.