Fix the bug of rendering post content with Swig, which is a template eng...

Fix the bug of rendering post content with Swig, which is a template engine that reads all '{{}}' as a variable.

A solution to  https://github.com/hexojs/hexo/issues/925 https://github.com/hexojs/hexo/issues/924 https://github.com/hexojs/hexo/issues/921 https://github.com/hexojs/hexo/issues/916 https://github.com/hexojs/hexo/issues/910

I didn't test very carefully, please check if it works.